Yeah, see France has this law about it being illegal to justify war crimes or something, so it's possible that von Trier saying that he "sympathize[s]" with Hitler is in violation of that. Oops!
After the initial uproar, von Trier did say that he was just being a jokester and of course he doesn't like the idea of World War II or of killing Jews and everyone else. He was just, y'know, doing lame promotional work for his movie at stuffy old Cannes and he wanted to have some fun with the uptight Frenchies. And now those Frenchies are kind of just proving his point.