He likes to collect them from his victims.
It's a nice reminder, a reminder that he would do anything for his Izuku and mostly so he can count down how many people he's killed for his angel.
He keeps them in a bag under his bed, kinda like the tooth fairy. He hated that.
When Izuku found his bag of teeth because of course he would, he knows almost everything about him, even when Katsuki tries to hide it for his own good.
He had called Katsuki the tooth fairy. Not lord explosion dynamite god, the damn tooth fairy.
How uncool.
So now, instead of 'Kacchan' he is now nicknamed the tooth fairy.
No one else gets it when Izuku bursts out laughing when he says it, doubling over at the angry expression on Katsuki's face.
"The tooth fairy! Get it?"
No one gets it. Katsuki doesn't even remember how many times he has had to tell Izuku this.
no one gets the joke so what's the point in saying it, it doesn't make sense.
Izuku just smiled and laughed.
"That doesn't matter, because the joke isn't for them. As long as you know what I'm talking about, it's a fun time Mr. Tooth fairy-"
That gets him a smack upside the head. Still, he bursts out laughing.
So now when he's getting rid of people who touched, looked or tried to get with Izuku, he can't go one session without:
Izuku rushes downstairs to the basement, his footsteps thundering on the steps. The girl sobs and screams louder.
"Help! PLEASE!"
Thinking its someone on their way to save her. Nope. instead, she's met with an idiot, the same idiot she liked and the same idiot that belongs to Katsuki alone.
He grins from the railing of the stairs, already trying to contain his laughter.
"Sorry to bother you guys but-"
He holds in another laugh as Katsuki gives him a done look, hands on his hips.
"Be careful because- Bahaha! Mr. Tooth fairy is gonna put your teeth in a bag and give them to me as a gift! BAHAHA! Do you get it? Mr. Tooth fairy? Because-"
He bursts out laughing again, leaning on the railing with his stomach. The girl sobs and Katsuki stares, then turns away from a still laughing Izuku.
He glares at the girl.
"Don't worry bitch, you won't be left out on the joke in a couple of seconds."
Katsuki holds up his pilers, getting them really close to the girl's teeth.
Katsuki hears Izuku laughing all the way back up the stairs and rolls his eyes. He wears a fond smile and mutters to himself.
"What an adorable idiot..."
He starts to pull out the girl's teeth.
