Seth Carter said:
Doomguy doesn't get transformed because Willpower/the crypt shielding him/angelic magic.
That and he's the Doom Slayer ... and the player avatar through the game. That being said arguably he basically gives the tools to humanity's future doom right back into the hands of the one that will bring ruin to all.
Hayden doesn't transform because he's a giant ass combat (or at least combat capable) mech rather then an organic human. None of the upgrade drones or VEGA transformed either.
Of the those explanations, Hayden's is the one that makes more sense (although not why he didn't ever make any more combat mechs as some kind of safety measure rather then apparently useless Praetorian guards, particularly after the trip into hell that only he survived to get Doomguy).
Hell having competing factions (while not directly mentioned, the intro voice seems to be on your side, which is also whoever recorded the Doomslayer testaments that seem somewhat in praise of him) isn't unlikely, and Hayden maybe aligning or being manipulated by one is possible (something gave him the idea to go build a portal to hell somehow). But I'm skeptical that he's directly one.
It's been years since I played the Doom games, but VEGA is an AI .... and I'm not saying that Samuel Hayden is Satan specifically ... more like a future vehicle or vessel that is being manipulated by a powerful demonic force. In the same way Pierce was. It's no less telegraphed by the antagonist in Doom 3 and it feels about as mature or subtle as id Software will allow with the franchise. And with good reason, Doom has always hada fairly straightforward 'moral coding'.
Demons go explode, corporations will doom us all.
Like ..., let's just picture the absurdity of the Doom universe. People learn Hell, or at least some part of the romanticised ideas of itbeing a transcendental plane filled with the suffering of the damned and demons, is real. And rather than being blown away by the philosophical and theosophical ramifications of that
capitalists try to mine it.
It's gloriously tacky, unadulterated black and white morality that ultimately speaks to humanity's worst impulses.
In that fashion I feel like it's telegraphing the fact that Samuel Hayden will eventually be the embodiment of the worst aspects of this demonic war is roughly on par with tone and prior takes on this absurdist mess.
I reckon it's even money at this point. After all, it totally works that by some design Hayden was working with DG solely to get the Crucible back. And I could totally see why a major demonic faction would want to see that come to fruition.