Sans is thinking every one of his friends and his brother getting killed by Frisk, Hiding in the Corridor to meet Frisk to judge and try his best to punish Frisk for going genocide route and reset.
Sans let out a sigh, and thinking that why Frisk wants to go to the genocide route? When Sans was thinking he got corrupted by the footstep coming to the end of the corridor.
Sans saw Frisk coming closer to the Thorne, Sans is judging Frisk from a distance and he knows that something is different about Frisk between this timeline and the other timelines he fought before.
Sans Shortcut to block Frisk from coming to the Thorne.
"So there you are Sans, I was looking for you everywhere," Frisk asked with a Smile.
"You were Looking for me kid? Heh... I must be popular. And just looking at ya, You've been really busy, huh?"
"So, I've got a question for ya. Do you think even the worst person can change. . . ? That everybody can be a good person if they just try?" Sans asked.
Frisk step toward.
"..."
"Heh heh heh he, It seems that was a stupid question. All right. Here's a better question. Do you want to have a bad time? Cause if you take another step forward. . . You are REALLY not going to like what happens next."
...
"...pfft" Frisk burst out of laughter when Sans said that.
"Sans, Do you think you can scare me. " *Laughs*.
"If your so strong you have just protected everybody or at least take some time for the monsters to escape in the underground like Undyne. But no, instead you nowhere to be seen while I'M KILLING EVERYONE. that's included your dear brother papyrus." Frisk replied happily to sans.
Sans almost lost temper when Frisk said that to him. but manage to control himself.
"Heh... You're really good with words kid, even though that you killed everyone without even talking to them. Everyone has reason kid" Sans replied with a scary face.
...
"Welp. Sorry, old lady, This is why I never make promises."