sans didn't know why but he could feel the presence of a being watching him from outside time and space (almost like It wanted to watch over him) sans "ok that's not ominous at all" as time went by frisk eventually arrived, sans "so the psychopath returned. you were so late I almost thought you decided to stop which would have made what I did pointless. glad you decided to continue, because now it's time to give you an eternal mad time" frisk now began feeling afraid of sans (he never mentioned a 'mad time' before. let alone an 'eternal mad time' this means he truly is no longer the 'same sans we knew' and who knows how much his madness changed him from who he was. and either way he's grown in power and determination, I can even feel my control over the world slowly fading) 

chara (this is an interesting change from the normal timelines of this world, but he took MY levels and MY entertainment. it's time he learned of the consequences) the body of frisk walked closer to sans 'a feeling of dark megalovaina filled the underground' sans "I guess you also want this fight to begin. just remember after the fight it was you who chose this option. and there was a small chance we could have just lived together in peace in the underground or something, if you just made a genuine attempt to end this senseless cycle of genocide" frisk accidentally showed sans just how nervous she was from the sweat appearing on her forehead

sans "oh is the demon just now beginning to be afraid of the consequences of her actions?. It's too late for any form of apology. besides. you are the reason why I had to chose this path in the first place, so no matter how many times you reset the timeline I still will continue on this path" as the battle began frisk and chara were having a lot of trouble avoiding attacks. And decided to check that stats of sans (level 45. Attack 666. Defence 777. Hp 600. 'The underground's spirit of vengeance' 'the embodiment of revenge' 'the divine murderer with demonic power' 'the first vampiric skeleton'). It seemed to frisk and chara the little hope they both had for victory had been shattered into nonexistence. The certainty of there defeat was proven even more true as after they managed to land there first hit on sans, the next hit he landed was shown to have a health draining effect restoring him back to full health

It felt like the entire universe was going against frisk and chara. And then frisk noticed something that she nearly forgot about that would end the fight faster then even chara would have thought. Frisk (I can't believe it. Not only is this fight near impossible to win. but chara and I have already ran out of healing items. and to top it all off not only does he have nearly unavoidable attacks but the only chance to win is to outlast his stamina, which seems to have been enhanced from his raw determination). After the relaxation that this fight was a war of attrition that they were losing. frisk and chara were trying there best to fight for even the smallest shard of hope. but were denied even that. Because like it or not they had to accept the reality of the situation. They are currently only 1 hit away from death at the hands of sans, who through shear wrath gained power that surpassed them both in almost every way

As sans landed the last attack on frisk he quickly went over and absorbed the souls of frisk and chara. thus ending the cycle of genocide. But now he had to deal with something new. As far as sans knew before this the genocide was only from frisk and that chara was in an afterlife, and there was also the fact that the being he sensed was watching earlier decided to reveal herself and it turned out to be a version of frisk?