I believe that in the Genocide Run, Sans does not die after he walks off the screen. Though you may hear the dust, he does not really die. When you go to the medic place where you have to sleep, the little guy says this, "Mom says that sleeping can make your hp higher than it is.", or something like that. And what does Sans do most of the game? Sleep. Another hint is that after you "defeat" Sans, go check the kill bar. It should have improved to 106 if you have killed Sans, but it remains at 105. Another clue. Also, Sans is quite different from other monsters... you may have been played at by Toby Fox when you defeat Sans and gain EXP becuase Sans is different. You gain EXP becuase you defeated him, not killed him. Imagine how much EXP and gold you would earn when you really kill him! But, don't start hating, because this is just a theory I recently figured out on my own. Feel free to give your own opinions on this theory though! Just make sure to say that in a friendly way, not in a mean way please... I might get it the wrong idea and then block you (if that is even possible but I hope that it is so that I can block off some haters hehe) Any ways, capiche? Good, great, have a great day.