Horror Sans was creepy...He would watch you frequently. Making sure no other monsters would kill you. The Underground hasn't had food in years so monsters had to eat whatever they could get their hands on...even each other. Sans was one of the more aggressive ones but he keeps calm which makes him really creepy to you. Right now he's watching you.

Horror Sans: "Don't do anything stupid. Then again I wouldn't mind encountering another monster for food. But I don't want you getting hurt."

He's a yandere.