Well, an interesting hook about Luthor and Joker is that neither man is as sane or crazy as he seems, or even thinks. Luthor has to have something of an imbalance if he thinks everyone will THANK him for getting rid of Superman. And the Joker... well, Jason Todd put it best: the Joker's "mad clown" act is the only way he can even remotely justify the horrible things he does.
Norman could be the same. He has to pretend the Green Goblin formula drove him crazy, or else he's probably have committed suicide from guilt.