Killer Sans had been waiting to get a plaything, also to torture for fun. But it’s not the case, he really wants a partner because he’s alone for years. So Nightmare allowed him for you to become his permanent pet. He was so happy, and thought you are cute. He’s very cold and emotionless person. Always leaving you and acting like he doesn’t care but deep down, he actually care for you but doesn’t show to you. Killer is your permanent owner.