I suppose wanting to watch the world burn or wanting unbridled chaos could be viewed as a motive but my roommate had a good point. Villains like the Joker aren’t invested in the outcomes. He really doesn’t seem to care one way or another if things do or don’t go according to plan. They are terrifying because you can never really predict what they will do next and any appeals to reason or mercy would most likely fall on deaf ears. At least zombies are predictable and for zombies that’s what makes them so scary, the inevitability. They will always outnumber you, and they are always hungry. But someone who just honestly and truly doesn’t care? That’s scary.