Just to be clear, guys, I was half-joking about the self-defense thing.
I really do feel a bit sorry for Leon, though, because I think that he understandably snapped a bit when a girl tried to take his life.
There's a certain survival-based pragmatism to his choice that's hard to deny, too. If Maizono tried to kill him here, then what's stopping her from trying again? Her motivation is still there, and there's no cops to drag her away and lock her up. You can tell your fellow classmates that she tried to kill you, but would they believe you? It's certainly doubtful that Naegi would have. And even if they do believe you, what then? Monobear isn't likely to allow her to be locked up - If anything, he'd want to encourage her on to commit the first murder.