The Anti-God was actually The Anti-Monitor
The Anti-Monitor was Not Quite Dead after Earth-1 Superman chucked him into a star and triggered the Supernova, but was still hovering on the brink of death. In a desperate attempt to heal himself, he created the green goo Space Satan to find a way to open a portal to another universe so he could devour it to heal himself and regain strength so he could go for a second round with the DC Universe, but he was so badly crippled at this point that even perfectly ordinary scientists and a priest could (barely) thwart his schemes.
The students possessed by the zombies weren’t killed and could have been turned back to normal as well.
