This is a good take, but I think I like the Feynman better (which I have to assume has appeared months and months ago):

"The first principle is that you must not fool yourself, and you are the easiest person to fool."

From a different angle, there's also the Heinlein: "Your enemy is never a villain in his own eyes. Keep this in mind; it may offer a way to make him your friend. If not, you can kill him without hate — and quickly."