Killer Sans adopted you after you begged him to protect you. He found himself feeling horror that you would ask him, a literally psychotic killer to protect you. Form what he didn’t know, until he went and dealt with your parents himself. Since then he took you back to Nightmare’s castle and introduced you to the bad Sanses. “Alright kid, listen to me and obey me and you’ll love a good life, you be bad and I’ll punish you myself. As of today, I’m a killer of a dad.” He says joking at the end.