(Au- Katsuki is insane, Not abusive but bad parent Mitsuki. Warnings: insane Katsuki, dubious consent,blood, implies sexual content. Idk how to explain it you'll have to read yourself. I went full adhd zone out on this. Sorry if it doesn't make a lot of sense it does to my adhd mind but this is a bit of a weirder and darker one.)

When kids are selfish, parents don't stop them. Maybe that was the problem for most kids.

Maybe the problem for Katsuki was that his mom did stop him.

Katsuki wasn't allowed to be selfish at any time. When Katsuki was four, he wanted a cat even though his father was allergic. He got hit on the head and was told not to be selfish.

When Katsuki was nine he wanted the last All Might toy and grabbed it from the shelf first. But another kid tried to grab and started crying to his mom. Mitsuki made Katsuki give him the toy.

Maybe if Katsuki had been allowed to be selfish then, he wouldn't be so now.

Izuku was the opposite. Katsuki can't remember a single time the greenette was selfish. Maybe that's why Mitsuki loved him more.

Katsuki watched, even when Izuku and him didn't get along. The blonde watched as the guy he's known since childhood smile kindly and offer the last of his dessert to a kid who didn't get any in their lunch.

The blonde watched as Izuku offered his notes to a girl who forgot to take them.

The blonde watched but he didn't understand. Anytime Katsuki was forced to give something away he'd always have a hot ball of anger rising in him. Izuku didn't have that.

If Katsuki was fire, Izuku was the rain that came after. While Katsuki gave everything up with yelling and screaming, Izuku was the one who offered it in the first place.

Maybe that's why Katsuki couldn't drop Izuku.

No matter how hard the blonde tries throughout his life, he always found his way back to the greenette. Katsuki lost more friends than he could count except Izuku. No matter what he did, the greenette always returned like a boomerang.

Maybe that was what triggered Katsuki's obsession.

No, it didn't start as an obsession. It was a mild interest at first.

Seeing what he could do, push the limits to Izuku's selflessness.

Threats, violence, destroying his stuff, anything, Izuku forgave him almost the exact moment it happened.

That was like a drug suddenly.

It was Katsuki's first taste of selfishness and it was addictive.

Being able to take whatever he wanted when he wanted it. The amount of power he felt at his finger tips couldn't be compared to any quirk available.

Izuku was completely his.

Then they entered UA.

Katsuki saw Izuku's quirk and he should feel happy.

Katsuki saw Izuku with friends he'd follow around and he should be happy.

But it felt like someone was force feeding him sewage as he watched Izuku with other people.

Izuku wasn't the same weak nerd that was always following him and giving up everything for Katsuki.

Suddenly Izuku was trying to give his life for the school. For people that didn't even matter!

But Katsuki fought beside him. Izuku's life wasn't his to throw away like that. Katsuki had to protect what was his.

If the gods hadn't wanted Katsuki to be selfish, they wouldn't have let him met Izuku because it was already to late. Izuku belonged to him. Izuku was Katsuki's.

Katsuki took what he wanted when he wanted from him.

Or that's what the blonde thought at least.

"I won't be your punching bag anymore!"

That sentence turned everything Katsuki knew upside. He didn't even remember anything that happened after.

He doesn't remember if he screamed or yelled. He remembered going to his dorm and destroying anything that could be destroyed.

It stopped when he got to an old photo album the old hag had given him. A picture of his and Izuku smiling was the front cover.

This wasn't fair. If Izuku didn't want to be Katsuki's he should've fought back a while ago. Why now? Why when Katsuki had already gotten so used to being selfish? Why when Katsuki had already decided that the greenette would be his so long ago?

It was like Katsuki was in a trance.

The blonde walked out and went over to Izuku's dorm.

The greenette wasn't awake why would he? It's been a week since they last spoke and it was nearly 2am.

Katsuki reached for the door knob and it was unlocked.

The brief anger over the fact that Izuku was that careless over a life that wasn't his to throw away resided after a minute when he entered the room.

The room was dark, relying on the sole window letting the moon in. The posters on the wall casting shadows across the moon lit room.

The only sound was the quiet breathing and the click of the lock behind Katsuki.

The blondes eyes scanned the room for the bed.

The room had a blue like glow from the moon and almost seemed empty and eerie.

But it wasn't, Izuku laid sleeping in the corner looking peaceful as ever.

Katsuki was slow in his movements. His mind repeating to take what he wanted, urging him to be selfish.

Katsuki didn't plan anything so for a while he just sat and stared at the greenette sleeping.

He kneeled beside the bed as he watched. The shadows slowly inching their way over Izuku's face was the only indication that time was even moving.

Katsuki sighed as he watched, piecing together a plan but all of them failing when he he glanced back to the greenette.

He could beat him up. Tell Izuku that he already belonged to Katsuki, that he had no choice. But that wouldn't due. That would only scare him away the moment Katsuki wasn't watching.

The blonde let his hand lay gently on Izuku's neck. The skin was cool against his hand and the blonde squeezed it a little. Just seeing if it was easy to wake the greenette up.

Izuku hummed a bit and turned towards Katsuki stirring from his sleep.

The blonde knew he should've hid. That was what a normal person would do in this situation.

But the blonde just stared as emerald eyes opened up and stared at him.

"Kacchan? What is it?"

Izuku went to run his eyes but the blonde other hand slapped it away with a huff.

Izuku got the hint and just glared, obviously trying to figure out the mystery still half asleep.

Katsuki's thumb rubbed along Izuku's neck thoughtfully. Trying to think of how he could keep Izuku.

When he stared up at the emerald eyes he saw confusion. But the greenette wasn't leaning away. Quite the opposite.

Izuku hummed in question but his body was relaxed.

That's when an idea came into the blondes mind.

Katsuki used his other hand to gently push Izuku back onto the mattress he had been laying on. Without any complaints the greenette listened.

Katsuki felt it. The same power of control over the greenette ray the was addicted to. He felt it anytime his hands made contact with Izuku.

So Katsuki leaned forward, half on top of Izuku, and kissed him.

The blonde ain't going to say that he always loved him, he didn't. He wasn't even sure if what he felt now could have been considered love. It was more like needing. Needing for him to stay by his side at whatever it costed.

The greenette tensed but relaxed quickly as he kissed back. Arms came up behind the blonde and pulled him closer.

Katsuki pulled back as Izuku looked at him breathless and now fully away and still trying to figure out the mystery that was happening.

Katsuki's mind urged him further. A voice whispering in his ear that he needed to make himself irreplaceable in Izuku's life.

So Katsuki went further, not saying anything to Izuku as he kissed the green's neck.

He felt hands tangle themselves in his hair and pull him up to face the bow flushed greenette.

"Are you jealous?"

The blonde stayed quiet as he tried to get back to what he was doing.

"Because of what I said about not being your punching bag? That seems a bit-"

"You're mine."

Izuku stared at the blonde.

Katsuki stared back, almost angry as the grip on Izuku's neck tightened a little.

"If you didn't want to be mine you shouldn't have been so nice! You've put up with me all these years why now?! I'm treating you better than I have for a while and now you're saying you won't stay beside me?! I don't get it! If you didn't want me to be selfish why would you wait! Why would you-"

A hand gripped Katsuki's wrist tightly and pried it off the neck.

Katsuki stared, horrified, as he watched Izuku coughing and holding his neck. An ugly bruise now covering the skin. The blonde looked at his own hand as if it had betrayed him personally.

Izuku got over his coughing and heaving fit and stared at the blonde who had scurried into the dark corner of his room.

The greenette tilted his head, very much confused but one thing was clear in his mind.

Izuku stood up and walked over to the blonde calmly. He kneeled beside the blonde and gently pulled Katsuki to lean on him and he played with the blondes hair.

Around the time the sun was just about to peek into the window is when the blonde looked up at him.

Eyes puffy and tears streaking down his cheeks.

"I-"

Izuku kissed a tear at the side of Katsuki's eye.

"I know, you didn't mean to. I forgive you Kacchan so calm down."

The blonde was careful when he pulled Izuku in front of him so he could lean across the greenette's back.

Izuku hummed as he felt Katsuki braid his hair.

The room was silent except for quiet breaths as both of them took time to breath.

"You... you don't get to leave me. You're mine Deku. It's already to late to leave. I was horrible in the past but then when I treat you better? That's when you decide too-"

Izuku leaned fully against the blonde and held his hand.

"Im yours. Don't worry Kacchan. You've already been forgiven for all that so-"

Izuku didn't even realize what was happening until he felt a sudden, and very powerful, sense of pain in his shoulder.

An arm was placed infront of his mouth to stop his scream.

In revenged, Izuku but down, tearing the flesh there as tears gathered in his eyes.

When the pain subsided the a dull pain, he released the now bloody arm.

A hand came up behind him and wiped at his tears  and something brushed over his newest injury.

Katsuki lazily licked the blood the came from the bite that was sure to leave a scar.

The greenette was breathing heavily which only prompted Katsuki to kiss the green's nape and rub his side with his non bloody arm.

The wound stopped bleeding after a bit and Izuku stopped shaking like a leaf.

The blonde smiled as he kissed the napes of the very confused boy in his arms.

"Don't forget you're mine."

Izuku stared at him for a minute or too. Looking from his eyes down to the ugly mark on his forearm. Then he turned to the blonde slowly.

"I don't understand I thought this was a confession- I- why- You bit me!"

The blonde laughed as he kissed the mark on his shoulder.

Izuku was still very clearly trying to piece together a puzzle he didn't see.

Katsuki just smiled. Now Izuku couldn't leave him. He'd know and rennet him every time he looked in the mirror.

"Take it as you want Deku. Take it as a confession or threat."

The greenette flushed as he tried to understand.

Katsuki leaned in and laid a gentle kiss on the corner of Izuku's mouth where his own blood stained.

"But you are mine. Wether we're married of enemies, you are mine."

Izuku was left blushing and struggling to understand what was happening even after Katsuki left.

When in class, Izuku still didn't understand. Everytime Kacchan's eyes lingered he could feel the mark on his shoulder like it was trying to burn through the cloth. Katsuki only smiled then continued what he was doing.

Later in the night, it was Izuku who had broken into Katsuki's dorm this time.

Unlike Izuku though, Katsuki was wide awake and smiling as Izuku wordlessly climbed into bed with him.

"What are you doing Deku?"

It was teasing, Izuku knew it was. But he still hugged the blonde closer, burying his nose in his chest as he hummed.

"You but your boyfriend like a psycho so you can get over me breaking in and cuddle me."

Katsuki smiled as he complied. He still wasn't sure if he felt love but he could feel himself getting high off the power he had as he stroked down Izuku's back, fingers lingering longer than they should on where the bit would be had he not covered it.

After graduation they moved in together. When they got married at 21, they celebrated.

Katsuki still felt the same high every time he had Izuku under him, only it had spread. It used to only show up when he was making Izuku do something or getting his way. Now the feeling showed up whenever he and Izuku so much as talked. The blonde craved that feeling. Craved the feeling of being unstoppable by only talking with Izuku. He was thoroughly addicted.

He was addicted to the flush that settled on Izuku all the way to his chest.

He was addicted to the freckles on his body.

He was addicted to the way Izuku's head fell back when he was feeling good.

He was addicted to Izuku's voice calling his name.

He was addicted to Bakugo Izuku.

Now Izuku was his forever though. He was truly selfish when he stared at the greenette and vowed that he would never leave him.

He was selfish when he first but Izuku.

He was selfish and Izuku was to selfless.

But Katsuki smiled as he pet Izuku's hair.

His addiction and the object of his selfishness was one and the same.

But he was fine with that. He was fine with being selfish if it meant he could keep Izuku. He was fine with being addicted to Izuku.

This was an addiction that he knew he wouldn't be able to get over so why try? Especially when Izuku was more than happy to feed the addiction himself when the greenette did absolutely anything.

Katsuki hummed as he laid next to his husband and held him close.

"Now I can be as selfish with you as I want stupid nerd."

Izuku hummed as he cuddled closer.

"I'm still going to work next week."

He heard his husband loudly complain and throw his fit but it only made Izuku laugh.

Maybe Katsuki didn't start out loving him but Izuku could see it in his eyes. The blonde did love him even if Katsuki didn't understand it.

So Izuku didn't complain when the blonde swore and held him close. He didn't complain when the blonde tried multiple plots to get him to stay home from work to be with him. He didn't complain when he sat and Katsuki threw a fit because Izuku didn't give him a morning kiss.

Maybe it was dumb, maybe was just stupid to ignore all the blatantly obvious signs that Katsuki wasn't the best choice for him but he couldn't help it.

He's always been addicted to his Kacchan.