According to the game's lore, this is the result of the Seraphim giving the Doom Marine the ability to absorb the souls of those he killed.

Destruction Empowerment: The most unique thing about the Doom Marine is his ability to grow stronger and become even more tenacious with every demon he kills.

These abilities are all augmented by the Doom Marines Praetor Suit and are used when fighting against the Demons of Hell. His strength is sufficient to grapple and overpower superhumanly strong demons in hand-to-hand combat, and perform gory 'Glory Kills' to ensure their complete destruction. He outmaneuvers his opponents with great speed, avoiding their attacks, and yet is capable of enduring the hits that the Demons are able to land.

Superhuman Abilities: The Doom Marine has shown to possess immense physical Strength, Endurance and Durability, and superior Speed and Stamina.

This makes him an invincible and unstoppable force to be reckoned with (though it depends on the player's actions).

Badass Incarnate: Having become an icon in gaming, and battling the forces of Hell for decades, has earned the Doom Marine the status of being an absolute badass.

He also demonstrates this during the penultimate mission, when he decides to take a back-up of VEGA's code before destroying it, despite not being asked to do it. Unlike some of his earlier appearances in the series, he shows a little more emotion, evident when he angrily destroys the communication panels that Samuel Hayden tries to use and ask for assistance from the marine after the explanation of necessary measures and sacrifices being taken (shortly before the Doom Marine sweeps his vision across several human corpses in the room). Over the bodysuit, he wears a bulky dark-green armor which includes a green chestplate with white linings, shoulder pads, elbow pads, arm gauntlets, a brown utility belt wrapped around his waist, a thigh-high armored boots with bulky knee pads and shin guards, and wears a round helmet with a bluish-green visor shield. The Doom Slayer has a large, muscular frame and wears a brown full bodysuit.

Quake champions doom slayer