WARNING: The following contains spoilers for Dark Ages #5, available now from Marvel Comics.
Unlike many other post-apocalypse stories, Marvel's Dark Ages series has shown that the end of civilization doesn't have to be the end of humanity. Amid the ruins of a world deprived of man-made electricity following a battle with a world-killing machine known as the Unmaker, the heroes of Marvel have risen from the ashes and built an intercontinental network of thriving city-states that have given countless people a chance to begin their lives anew.
Unfortunately, many of Marvel's villains have rejected this new world and the clean slate it'd presented to them by joining the empire that the X-men villain Apocalypse has created in Europe. Among them is Zebidiah Killgrave's Purple Man, whose pheromone-based mind-control abilities play a key role in Apocalypse's plan to claim the dormant power of the Unmaker for himself and destroy the post-apocalyptic utopia that the heroes fought so hard to create. However, Dark Ages #5 (by Tom Taylor, Iban Coello, Brian Reber, and VC's Joe Sabino) implies that Jessica Jones' arch-nemesis might be having second thoughts about Apocalypse's plans, and he may end up being the only one who can stop the genocidal mutant warlord.
While most of the heroes and villains capable of sensing the awakening of the Unmaker reacted with terror, Apocalypse became enamored with the raw power that the cosmic killing machine possessed. Taking advantage of the chaos that engulfed the world after Doctor Strange sacrificed himself and the world's ability to generate electricity to stop the Unmaker, Apocalypse conquered Europe with an army of supervillains, forcing its enslaved populace to remake the continent in his image while he formed a plan to reawaken and seize control of the Unmaker. To achieve this, Apocalypse began kidnapping scientifically-gifted heroes like Mister Fantastic and forcing them to create new methods of generating electricity. As Iron Man discovers after being kidnapped by Mystique, those who resist Apocalypse's will are forced into submission by Purple Man, who uses his mind-altering pheromones to rob the scientists of any inhibition while preserving their brilliance.
Given his well-established habit of forcing people to do his bidding, it makes sense that Purple Man would side with Apocalypse, having seemingly risen through the ranks of Apocalypse's empire to become his second-in-command. However, as Apocalypse's plan to awaken the Unmaker nears completion, it seems like Purple Man has begun having second thoughts, voicing an objection after Apocalypse forces him to accompany the enslaved heroes into the cavern where the Unmaker's dormant body sleeps. Although Purple Man's defiance is quickly ended by a single glance from Apocalypse, his reluctance shows that the First One doesn't have the complete loyalty of his primary lieutenant.
While Purple Man has stood by Apocalypse throughout the seven years that have passed since the Unmaker's initial awakening, the recent developments in the war between Apocalypse's empire and the remaining heroes could give the self-serving villain an excuse to finally betray his master. Having forced a mind-controlled Quicksilver to kill both Human Torch and Okoye, Purple Man has painted a massive target on his back that powerful heroes like Invisible Woman and Storm will undoubtedly aim at during the upcoming battle, and Purple Man's cowardly nature might cause him to abandon Apocalypse once they have him in their sights. Even if he doesn't turn tail at the sight of the heroes, the possibility that Apocalypse could fail to control the awakened Unmaker has more than likely occurred to someone as intelligent as Purple Man, and there's little doubt that he'd be willing to help the heroes put an end to Apocalypse if his obsession with the Unmaker's power drives him to throw caution to the wind in his pursuit of power.
Although Purple Man has never cared for anyone other than himself, that self-interest could drive him to betray Apocalypse to save his own life. As the only thing keeping Apocalypse's enslaved heroes in line, Purple Man is an invaluable asset in the mutant warlord's plans, and relinquishing control over their enslaved friends and loved ones at a crucial moment during the battle could give Purple Man the opportunity to convince the heroes to show him mercy once the crisis is resolved. Regardless of how the assembled heroes may feel about Purple Man, a group that includes Jessica Jones and Luke Cage, their inherent code of ethics would likely prevent them from executing him outright, and Purple has shown that he considers prison to be preferable to death in the past.
Even though any "heroic" action that Purple Man might take in the upcoming battle against the heroes and Apocalypse will probably be motivated by self-preservation, it could be the deciding factor that determines which side wins the war. Regardless of the motivation behind it, Purple Man deciding to betray Apocalypse would technically make him Dark Age's greatest hero.