As someone who is attempting to write suitable dialogue for similar, it's not easy. Especially for showing differences and shades of evil. Megalomaniac, selfishness to the extreme, chaotic to the max, self-delusion, etc.
Self-delusion is a big one though. "The greater good" bullshit that justifies almost anything. The lie one tells oneself the "I'm the hero in all this."
...sorry..rambling.