Board Thread:Questions and Answers/@comment-33450064-20171022193730/@comment-29936979-20171023001737

The player is so powerful in the Genocide run (due to them killing so much), that they can kill almost any monstetr in one hit. As for your second question, Sans doesn't fight the player in the pacifist route becuase he doesn't have any reason to, and he promised Toriel that he wouldn't. He only fights in the Genocide run becuase he wants to stop the player at all costs from killing everyone in the Underground.