I don't mean to say that if he's a spirit, he's pure evil (though I think Koh comes close, from a human perspective). My concern is that if he's a villain like Zhao or Ozai, who did terrible things and we were not encouraged to feel sympathy for (unlike Azula, who was made sympathetic by glimpses into her emotional frailties), and then it turns out he's a malevolent spirit, it creates this kind of distance between what he does and what human beings are capable of doing, in the same way that we separate ourselves from murderers and other people who do horrific things by calling them "monsters."

If it turns out he's sympathetic - like, if his story about his family is actually true and not the propaganda bullshit I think it is - then I'm not bothered by Amon being a spirit. But if he ends up being both unsympathetic and nonhuman, I would be uncomfortable.