I vote for Zulu as the most psychopathic of all. Aztecs have a real attitude problem also, but are not as deadly. As for Germany, I recently whupped them early in the game, leaving them only Berlin which I surrounded and culture-bombed for the next 2,000 years while I hermetically sealed their borders and didn't let the settlers out. Bismarck was hopping mad, but knew there wasn't a thing he could do about it.
Eventually I decided to make nice with him; I was very persistent about being super-friendly (gave him a little free money every turn; when I gave him free iron he finally cracked a smile), and guess what! By the end of the game his vote elected me Secretary-General and I won! So Germans can't be all bad.
But Zulus are nothing but bad news from start to finish. At best they're greedy bullies (Gimme stuff before I stomp you!), at worst they're dangerous maniacs.