What if? When you do a Post-Genocide True Pacifist Run, everything is the same. BUT as presumed, Chara destroys everything. You do another Pacifist Run, but afterwards, Chara taunts you for trying to figure out a way to prevent them from destroying everything. What if when you do it a third time, Sans' judgement is different:

... but you. you never gained any LOVE. * 'course, that doesn't mean you're completely innocent or naive. * just that you kept a certain tenderness in your heart. * but i remembered something. something that <playername> hoped i'd forgotten. * you... no, <playername> destroyed everything. you kept coming back. * you kept trying to prevent those disasters, * but nothing has worked, right? * what makes you think that this time it will be different? * i'm sorry kiddo, but i can't let you continue. * i don't want to have to do this, but... * i refused to go through that again. * i don't want Papyrus to go through that. *Sans fight starts * sorry Frisk, but... * it's a beautiful day outside. * birds are singing, flowers are blooming... * on days like these, kids like you, <playername>... * Should be burning in hell. Throughout the entire battle, Sans keeps apologizing, "sorry," and "please forgive me." Sometimes you can see tears coming from his eye sockets. If you lose and come back, Sans says: * please, kiddo, for everyone, leave. * i don't want to fight you, but if i have to, i will. [ACT] > [Plead] > * ... please. stop. the best thing for you to do right now is give up. [ACT] > [Check] > Sans, 1 ATK, 1 DEF. Doing the right thing. [ACT] > [Help(1st)] > You call for help. A tear runs down Sans' face. [ACT] > [Help(2nd)] > You call for help. Sans' feels his sin crawling down his back. [ACT] > [Help(3rd&On)] > You call for help. "stop. i'm doing this for everyone." [MERCY] > [Spare] > "sorry Frisk," [MERCY] > [Flee] > "* for both of us Frisk, please, don't come back." [FIGHT] > Sans dodges. "* i knew you'd resurface eventually, <playername>. why are you doing this?"