Aang had a deeply-held belief that using bending to take life is wrong. Even so, he certainly struggled with that belief up until the very last second in his battle with Ozai. Not sure what there you can't buy. It was a very understated moment, and something I'd like to see more of in shows like Naruto. Many anime don't seem to think that audiences can grasp "understated" behavior, so the characters are often stopping and spending awkward minutes discussing their motivations and feelings (even in the heat of battle). Even my all-time favorite anime, Full Metal Alchemist, often fell prey to the inability to say more with less.