During this run, Frisk is sick and tired of the monsters they "befriended" so they decided to betray them and kill them. This caused Frisk to snap and kill Toriel, who begged for her life and left Sans unscathed, just for him to feel the pain Frisk felt. They then killed Papyrus who asked for mercy. Then the list just kept on growing, despite Flowey's discouraging comments. Then Frisk fought Sans but didn't kill Sans because someone interfered and made sure Frisk would never see Sans again. Frisk then left Sans alone. Even when Frisk killed Asgore with Flowey looking at his death in terror. The whole genocide run, the only reason Flowey wasn't killed by Frisk was because Flowey was collecting the souls. At the end of the genocide, Flowey takes the 6 human souls and all of the monster souls and becomes Asriel. During this fight, you do something similar to "Saving" in the Undertale Asriel Dreemurr fight, except you make the monsters insane and break their spirits. At the end, Flowey goes insane, destroys the barrier, and kills himself.