One particularly horrible motive has come to me recently. Ron Rosenbaum's Explaining Hitler is the first, and so far the only, book I've seen which suggests that the Holocaust was a vast jest. Consider, if you will, the famous "Arbeit macht frei" motto greeting incoming prisoners at Auschwitz. Were the Nazis really going to allow the inmates destined for extermination escape and life if only they worked hard enough? And then, there's the classic, "We're sending the Jews to ... Madagascar!" (Cue cymbals.)
One definition of humour has it as the "ability to perceive, enjoy, or express what is amusing, comical, incongruous, or absurd." If you're in the process of committing genocide, and if you believe that the victims deserve their fate, might you not find it a bit of a stress-reliever to play on the incongruity between your victim's disbelief and your certain knowledge? True, this is a one-sided humour, but when does humour have to be fair to all parties concerned? After the genocide's over, you can't have the same fun that you used to--you run out of victims, for starters. There's still an incongruity between the knowledge that you share with the wider world and your public pretense, mind. Your genocide denial also runs the decided bonus of tormenting the survivors just a little bit more. What's the harm to you?