AceAltair
Sans the Skeleton has always been plagued with nightmares, promising death and destruction to all monsters in the underground.
It only wasn't until a certain human, Frisk, fell into the underground. Sans never did trust them, there was just something off. Ever since they arrived, the nightmares grew even more frequent.
So when Sans' brother, Papyrus, grew fond of the human, he knew he had to protect his brother at all costs.