Nightmare!Sans is, in general- sadistic. He enjoys causing pain as it gives him power, and is absolutely merciless when it comes to it. Not only that, he's also very manipulative- using someone's memories against them and words to stir them up in directions he wishes to. While he tells Cross not to kill anyone, he's given X-Event!Chara a chance to kill somebody, and when that fails, he kills Underswap!Chara himself. He even tries multiple times for Underswap!Papyrus to die.