Taking Sides

The term used in kabbala for the side of impurity is Sitra Achara which literally means the other side. This is because those impure forces tell you, “You are on the other side,” not the side of kedusha. This can lead a person to falling spiritually.

This explains the name Acher

(Elisha ben Avuyah, who became a heretic), meaning he made you an Acher, someone on the other side. The name Acher is associated with the term Sitra Achara due to his spiritual fall. He fell to "the Other Side" spiritually.