Doomguy is actually a post-doomer:

He knows that the mega-corporations are evil and that the world's future is hopeless. He knows he isn't going to save anyone.

But he keeps killing the demons, because it's simply the right thing to do. It's who he is. Why did he get sent to Mars in the first place? Because he spoke up about the injustices he witnessed in the Marines. And as every Marine knows, that's the worst possible thing you could do for your career trajectory.

But doomguy doesn't care. Because doomguy doesn't believe in other Marines, in corporate bigwigs and other cultists, in the scientists naively working towards enslaving the galaxy, or in ignorami such as you or I. Doomguy has been liberated from the chains of hope.

Now he lives a simple path, following his own dharma: Doomguy sees fucked up shit, and he fucks it up.

A true role model for those seeking reason in a life bereft of hope. Doomguy teaches us how to find meaning in the process, rather than the outcome.

Flowers can grow even in the most barren desert, and a Zen master can arise among even the most despondent. The demons cannot bear one with such control over the elements of their own faith and despair. And so, generation after generation, they taught their spawn to fear the coming of doomguy: The one who would overcome the trappings of hope and ego to selflessly deliver justice against his masters.

Only one who would sacrifice it all could destroy it all - to pursue ruin as progress - and only one who truly spites the mortal coil can offer such a sacrifice. Doomguy is the best of us.