HORROR!HORROR: EXTRICATE REVOLUTION
This concept goes based on the movie(s) Happy Death Day. Sans isn't the one who really remembers after death, but instead it's Undyne.
Horrortale version of Undyne remembers her own death as she wakes up before Horrortale's events. Curious, she decides to continue to try and plot against the human that's already currently in the underground, heading to kill the queen. She deliriously forces Alphys to use one of her older devices that traces a monster's memories. Knowing that Sans was one of the more violent monsters during the events of Horrortale (and that Undyne was the reason of it (not as though she really cares)), she brings Undertale Sans into the Laboratory to give back his memories as Horror Sans, though the adjustment was too unexpectedly quick, as he moreso ended up just loosing his mind over it, unable to handle all of the information. And due to the quick mind transfer, Sans was given the thought that the current human was evil (the player obviously was when this happened duh), so he ended up preparing, using whatever he could to his own advantage.
Horror*3 lore soon too I guess





