Horrortale ( Bone Apetit Version ) is just a Concept AU. No story, just a backstory that exists.
This is set on Post-Genocide run, which is why the monsters here are different than the original.
Sans is one of the main characters in Horrortale, he is very kind, but when it comes to killing, he would turn insane due to remembering genocide run, or remembering the time he attempted to kill Papyrus and Undyne. And kill Frisk.
He actually attempted to kill Papyrus and Undyne, but it didn't work. The reason why he would do this is due to insanity.
Because of attempting to kill Papyrus and Undyne, they both give a come back to sans, which result on sudden changes on his appearance.
His appearance is normal before, but the only thing that is different before is his colors has darkened.
Sans has gone insane because he tried to forget about the genocide run, his hallucinations make the situation worse, so due to insanity, he thinks Papyrus is possessed, and if he didn't kill him yet, he would kill Sans, so he get a knife and try to kill him. Papyrus then senses Sans, thinking his a intruder. So he dodges, get the knife, and almost stabbed him. But atleast he didn't kill his brother. Sans then go back to his senses, and apologized to Papyrus.
And it happened again, he has gone insane again, he hallucinated. In his perspective, the waterfall are full of blood, and dust. So he thinks Undyne, is possessed. He tried to find Undyne, and get the same knife. He almost killed her, but then Undyne senses Sans. and crack his skull with her spear. ( Atleast it didn't break. ) Sans then go back to his senses, and apologized to Undyne, but he felt "Blood" ( Ketchup ) going to his eyes, and fall. He try to ignore it as wiping it would keep it going.