I'm using Batman Logic. It doesn't really matter if he swung first, it's not free game at all. If you want to be a real hero you need to try and save everyone and that includes the "bad guys" of the story. You have to be better than the person you are fighting. Figure out a way to deal with the situation where everyone can be saved.
Batman can't go: "Well he had a knife Alfred! He swung first! I had to take him out!"
Nah that isn't the Heroes way, at least in my mind.
Depends on what kind of story it is, I guess. If it was realistic, it'd be free game after the option of stopping wasn't taken. Even more so when the bad guy starts attacking.
But since this series is all about being a TRUE HERO!™, it would go more towards what you suggested.
But I doubt that slash actually kills him anyways. This seems like a pretty child friendly manga and I doubt they'd add an on-screen death like that. I bet it'd get explained like: "Every superbeing has a natural energy-based shield!" or "The katana blade was set to stun!".