Many Genocide routes were played sans got sick of the human, he decided to start his own genocide to overpower and
kill the human. He would even
kill his own
brother - papyrus. However Papyrus already knew about this, so he tried to stop sans from this twisted path, sans didn't listen forcing Papyrus to kill him in self defense. Papyrus remembered all the timelines and the genocides, distraught, papyrus vowed to avenge his
brother and everyone else to give them the happy ending the human never gave... No matter the cost, NO MATTER THE COST. He went to waterfall and murdered the inhabitants including his friend, undyne who was the leader of the royal guard. He did the same to hotland and even killed the king when the human finally came. Papyrus was finally able to get his revenge and will complete what his
brother started.