Acharei Mos- Kedoshim

Why does Rashi 17,4 say that a person who shechts a korban outside Beis Hamikdash is like a murder that in that he pays with his life? There are tons of people who are chayav misah, what relevance does this guy have to a murderer? The Torah must be gaining something by the comparison. It can't be the Torah needs to show that its possible for someone to be chayav misah! Besides, a murderer gets Hereg and someone who shechts a korban outside the Beis Hamikdash only gets Kares! Why don't we compare him to someone who gets Kares instead?