I don't understand the protagonist's way of thinking - and I see it a lot in anime/manga. It's like they have a completely different idea of responsibility and self-defense.
I mean, I sort of understand, because the basis in Western philosophy was Western thinkers such as John Locke.. But if you kill someone trying to kill you, that's not murder. That's self-defense. And in this case, all he did was prevent one classmate from murdering another - and that first classmate had already beaten another classmate to death just a few days earlier. I don't understand how it would be reasonable for the protagonist to come up with the notion of "I killed him". Had Classmate 1 killed Classmate 2, wouldn't he also be thinking "I killed her"? It's almost conceited, this notion of taking responsibility for deaths in this situation.
In western philosophy, the notion is that we all have an inherent right to life - and to take that away is homicide. To do so willfully or negligently is murder, to do so accidentally is manslaughter. The victims do not lose their right to life, but instead, someone tries to kill another person, what really happens is the attacker loses HIS right to life. Thus it is OK to kill that person, because they've transgressed someone's right to life.
If our protagonist is right, then he isn't the only one who bears responsibility - he's not the only one who protected the girl, after all. That means that if he's a killer, then so are all of his friends.