Amon, because of his many years of madness and loneliness, is cruel. Sadistic, he's a frightening, morbid joker. He's debauched, impure, salacious, lustful.
Deep down, he doesn't hate humans. He's lonely, abandoned. He's controlled by his hatred of others and the pleasure of seeing them cry. He can smiles while saying the most horrifying and gory things.
He knows how to control himself and remain "normal" when he's pretending to be human, but we can still see that there's something wrong.