Strangely... I don't think that Kirishima is a natural psychopath, to me it felt more that he is neurodivergent.
How he is now, he is a total psycho freak, but this flashback shows me that he just is wired differently from others, and after some not-ideal stimuli (at least for most people... I saw that face on the last page, the little freak XD) his tendencies started to form quickly.
He clearly had good perception and quick information retention, so he learned things quickly, but his social abilities are... not stunted, more like... different.
I presume that after this first beating from Kodaka, things escalated, he being the little masochist he is, he must have let things go, but at some point... something happened, and surprisingly enough... I will bet it wasn't because he got tired of beatings, and more than Kodaka or someone close to him must have done something to someone else, and Kirishima may be socially different, but he does seem to value others well beings more than his, so whatever happened made him beat the crap out of the punks and after that... well, we kinda know the path that lead to his current self and situation.
What I mostly got from the first chapter of flashback is... if things had gone differently, I think Kirishima would have been a much different person now, a little strange at the edges but I think that with the right guidance, he would have been a much better person.