actually i think there was another part to it
after Zamorak stabbed Zaros in the back with the staff, Zaros still fought back and while he was strangling Zamorak he slipped and fell on the staff, thus causing him to fade. the other gods were made at Zamorak's Betrayal and Banished him, but he didn't stay banished and when he came back he was a god... anyways its also rumored that the elves (well part of them, and humans) are tying to bring Zaros back