I think not using AK when in most people opinion it's deserved is moral, yes. But I simply do not care how horrible Amycus Carrow is. You're entirely right that he's violent, cruel, and does terrible things. But so what? That does not make it okay for Harry to torture him. The spell Harry uses is not incapitating. It's not a fighter's spell that causes pain incidentally. Its entire purpose is to cause immense pain, and using that on another human being is reprehensible.

Besides which, it would be protective to tie him up, or knock him unconscious. I don't really consider it protective - or gallant - to torture someone when there are other, more protective options. And he's not a prisoner exactly, but he's outnumbered and frankly, McGonagall could take him down herself without breaking a sweat. There was no honour there.