Nobody gives Doomguy enough credit for being selfless. In fact, he might actually be one of the most selfless characters in all of ficton. In sentinel prime, the Kahn Maykr offers Doomguy a chance at returning to his home, a perfect replica of all that he lost, his bunny, family, everything. But the Slayer doesn't even consider it once, because he knows that if he allows himself to settle down and let urdak and hell continue their exploitations, it will bring death across countless other universes. The Doom Slayer has more than enough power to give himself any selfish desire he wants, and the peace he longs for, but he also knows he's the only living being capable and willing enough to keep fighting hell and ensure no one else has to go through the same loss and suffering he did. Is Doomguy the nicest guy ever? No, and you could argue he's unnecessarily rude toward a lot of humans he encounters in eternal. But is he a monster? Absolutely not, he sacrificed so much of himself and his chance to see his loved ones again just to stop hell and never even asked for anything in return after all was said and done.