People completely misunderstand the point of Doom: Repercussions of Evil. John Stalvern does not BECOME a zombie after the radio tells him that he is the demons. He was a zombie the whole time. In Doom continuity, the zombies are soldiers who are being controlled by the demons. The orders that he thinks are coming from Cernel Joson are actually the demonic influence over him and the demons whom he thinks he's fighting are actually his fellow space marines.
It's deep shit.