Ooh I now have an idea for this.
Je was meant to be used as a weapon in the Human-Monster War, but was stopped and doomed to an irrelevant position for the rest of eternity, and the Sans actually in Undertale is a clone made for his strength as a last resort in case it all goes to sh*t. He was later adopted into a family in the hopes that he wouldn't revert to the killing instinct of the original Sans if he was raised as a normal monster.