Through unknown means (sometimes explained as a “glitch in the code” of UNDERTALE), Sans is able to remember all the countless No Mercy Runs the Human has done. After trying all means he can to stop them, he ultimately begins killing other monsters in order to further this goal, trying to gain EXP and LOVE in order to be stronger than the human and best them. This includes Papyrus, whom he sees as a bust of with floating hands as a hallucination. Sans loses his sanity because of this, becoming no better than the human he sought to destroy.