He was never fully able to descend to Azula's level because he always valued at least his own people. This meant that once the propaganda and brainwashing was shown to be wrong, he was more quickly able to switch to the good side.
And reading the Kyoshi novels really puts that into perspective. Korra didn’t ruin Aang’s characterisation. It simply revealed the nature of the Avatar cycle. Even Yangchen created problems for the avatar that came after her.