Perhaps during a meeting between Hitler and Franco, Franco has minions jump Hitler and his aides and kill them.
Since the Nazi administration was not particularly well-organized, we start having "wars of assassins" between the other top Nazis and the regime collapses.
Franco and Hitler did not get along. Franco refused to get involved on Hitler's side, and, much to the latter's chagrin, helped 60,000 Jews escape to Spain. Hitler met Franco once and hated it so much that he said he'd rather have teeth pulled without anaesthetic than meet with him again.
Perhaps, to have Franco murder Hitler during a diplomatic meeting, Franco needs to get the idea that Hitler is about to invade Spain and overthrow him (perhaps b/c Franco isn't being cooperative--Hitler helped him get into power after all). Franco decides he has nothing to lose.