After countless genocide routes, Sans remembers bit by bit of what the human has done, ( of course it isn`t canon that Sans remembers it all. ) Then he took action, but it was too late, He wasn`t able to save the lady behind the door. ( Sans does not know the name of Toriel yet because this isn`t the end of Undertale. ) Later, Sans tells Papyrus not to try to capture the human, nor get near it, Papyrus was confused, he said "But why?" But Sans could not tell Papyrus because if so, he would want to capture the human more. So he lied, Sans said there was no human, it was just a monster that looked like a human, and he should stay home. Then later, Sans confronts the menace, He fought the human, but he wasn`t able to kill the human, He was getting tired. Papyrus couldn`t sit around all day in the house, he goes outside and sees Sans and the human, who is about to kill Sans, then suddenly, Papyrus runs to Sans, and takes the hit. Sans was so shocked, Sans was prepared to attack, but the human was nowhere to be seen. Sans was filled with rage, but due to his lazy nature, he couldn`t save the others, the only thing he can do is judge the human, as he is the royal judge. The human goes to the judgement hall, as expected, and this is where the fight began. @UTTSans