The Doom Marine was:
A man of morals; refused to open fire on a group of civilians because his CO ordered him to. He knocked out/killed his CO, never confirmed.
Became the sole survivor of an entire Mars Base, single handedly destroyed the entire invasion.
Went back to Earth. Demons invaded already, and he kicked their asses back into hell, and to make sure they learned their lessons, he jumped in with them.
Eventually found by the Sentinels, where he made a name for himself as the Slayer, and jumped into hell AGAIN and stayed there for eons (Hell time) and then was tricked by his own bloodlust and the hell priests sealed him into a cursed sarcophagus, forever sealing him.... Until an machine with the consciousness of an (more of less) angel released him.
Spawn is.... a dead cop with a vengeance. That's his whole back story. I think Doomguy wins in this bout