story if you want it (some things are up to change):

Papyrus suddenly remembered every reset.... and so started to think of all possibilities to end the never ending genocides, and the only possibility he found to end it is to kill everyone to overpower or at least become strong enough for the human to give up in an impossible fight. He will kill everyone in the ruins and after leaving it will slaughter some monsters in snowdin but bad timing after the few monsters he killed sans appeared and saw him killing a monster, at first sans tried to convince papyrus but he didn't want to hear anything and attacked sans, but at one point sans in self defense attacked him, it didn’t do much but made papyrus change his mind, he lift his arm up and pierce himself with some bones to kill himself, he did this because he couldn’t live with himself and in his delusional-mental state though it was better without thinking how it would affect his brother…before that papyrus explained his reasons to sans and suddenly he also remembered. So to not put papyrus efforts to waste and also having now remembering the genocides sans decided to slaughter the entire underground, he had some time so he placed traps all over the ruins and left, and after that slaughtered everyone one by one and placing traps on his path, at some point he’d be strong enough to kill alphys .Toriel and new home was a piece of cake. He will reach to level 20 and he will wait for the human in the corridor just before the last corridor. where he will make sure they give up or DIE. Sans during these events can picture papyrus in his head and both can talk together like they were in another dimension, Papyrus being dead would regain some of his mental health and apologize for not telling sans that he knew more than he let on and for attacking him, he would also tell sans about everything he’s been going through including timeline knowledge, they would also both talk about battle strategies, when he talks with papyrus in his head sans would feel like he’s in a safe place. He doesn't speaks much anymore, he's ''eaten from the insides'' by his regrets and also is speaking most of the time with papyrus in his head. If he ever speaks it's for either to talk about his regrets in a few words or it would be muttering giberish. The "traps" are actually random encounters, where a fight would engage and the player would have to dodge fast a bone zone by going up almost instantly, if they don’t they’ll lose a lot of health and couldn’t potentially die. When the player finally chooses to give up, Sans will take their body and with the justice soul will open the barrier, then he will bury all of the coffins of the fallen humans at the surface , and will also bury the humans body, sans looking at the sunlight will start to remove his hood.. but will stop, take a bone, break it, and point it at his chest….