Aang is far too innocent. He hasn't really lived through all that he's suffered. The death of the monks he knew, when most of them would have died of old age anyway by the time he resurfaced? A vague realization. The death of all the other air nomads? Too big. It can't really sink in. He never lived through the raids on the temples, never looked in the eye the ones responsible. It's easier to forgive the man who killed your mother when you never actually knew your mother.

Yes, exactly. As someone else mentioned, the Avatar State is a great way to have your hero kill people without having your hero be responsible for people dying, and without having to have Aang deal with killing people.