Oh for goodness sake. You have to be cruel to be kind. Harry could have killed the guy, and he's had unforgivable curses on him plenty of times. Carrow probably used it on people himself. Anyway, when you're fighting for your life, do you really have time to stop and think about morals of what you have do? Unless you're playing chess, I don't advise you to stop, sit down and construct a debate about the consequences of fighting someone.