He was not alone in this category of monster. Some of the most destructive monsters of all time were good looking, funny, smart and engaging. While you were admiring them they were not monstrous at all. They were wonderful, lovely, cool, special, easy to be around, fun.

Once they show you their monstrous side they are obliged to kill you. It is humiliating for them, the possibility they could be revealed as monsters. Better to take no chances, kill all the witnesses, kill them, kill anybody who tries to help a witness!