The human has Reset and done the No Mercy Route several times. No monster was ever able to defeat them. However, in the background of their endless rampages, Dr. W.D. Gaster had been watching it all. During the 31st No Mercy Run, he decides to appear before the human in the Waterfall FUN Event corridor. When the human tries to attack him, they’re simply knocked unconscious as Gaster leaves and immediately starts heading towards Snowdin Town. There he meets Sans, who knows a lot about how the timelines work. Gaster tells him everything the human has done. Sans doesn’t believe Gaster, who talks to him within the skeleton’s own mind. Gaster then shares the experiences of the No Mercy Routes, and Sans sees all the pain and misery the human has caused. Sans then decides that he’s going to stop them, no matter the cost. He’ll fight on, until his last breath.