Since DOOM Eternal launch date is in 4 days, why not a Knowledge Bomb be dropped on people.
To begin: The Slayer and The Betrayer are two completely different people, it has been confirmed that they are two different entities. So the line of him ''seeking vengeance against the dark lords who had wronged him" could mean that he seeks to kill ALL demons since they took everything away from him.(And since it has been confirmed that there's a multiverse in the DOOM games, that means that this is a DOOM Guy who wants to kill all of Hell for he could be from a universe where Hell took over, and doesn't want it to happen to anyone else.)
We know that he was a terrifying foe to demons, even before his Solo Crusade and making them **** their pants. Since he was a member of the Night Sentinels, the elite warriors of Argent D'Nur, he already was tearing demon ass. And it also has been confirmed he was a General of the Night Sentinels, which is why he bowed a knee to King Novik.(Also he IS human and all from Argent D'Nur are from some otherworldly blood, which could be why the Marauder called The Slayer an 'Usurper', he wasn't born in Argent D'Nur, nor Sentinel Prime.)
Another thing to note, is that The Slayer was about to be killed by Hell Dragons, but a Seraph appeared and rescued him. And another thing that happened before his Solo Crusade, was that he was trapped in the Void by one of the Hell Priests when he and other warriors went to destroy the Soul Forge, the source of the Khan Mayker's power, and turn the tide of the war against hell, which happened during The Betrayal, the time where The Betrayer sold out Argent D'Nur to get his son resurrected, but as the Icon of Sin. (I believe that this could be the main reason why Argent D'Nur was taken over, the Night Sentinels were stranded in the void, and couldn't protect their home, and so The Slayer was somehow able to escape, and then began saying **** HELL! by slaughtering ALL demons who stood in his way.
During his Solo Crusade, he was tearing demonic ass like a boss, and that was before he got his upgrade from the Seraphim, the highest order of Angel in Christian Mythology. And continued to kill them, and was getting stronger and stronger, and even when a Hell Titan, a champion of Hell, challenged him(this Demon has the might of all demons the Slayer killed before that point), and still lost(The Titan, not the Slayer) After that, he went and found The Wretch Who Shall Not Be Named, a demon who could've forged the Praetor Suit, making the Slayer even more powerful, unkillable, and unstoppable. So Hell had only one option; Drop their largest Blood Temple on him and hope it would work. And their plan did work, they took off his armor, and had him in a sarcophagus for thousands of years before the events of DOOM(2016), and when he got out, he was really pissed off, and began killing hell again. And to summarize DOOM Etenral: Angry Worker overslept and now needs to catch up on the work he was given.
And the Hell The Slayer Solo Crusaded through, was at its PEAK, meaning he took down an Inter Universal Conquering force, by himself.(While yes, he did get a blessing from the seraphs, that was after he was already killing demons like a mad lad.) So the Hell we fight, is NOTHING to what it once was, and even then, imps could fire balls of fire(anyone know how hot an Imps fire ball can get?), and that's the weakest demons. (And this is a Vastly weakened Hell, so we can assume a Hell at its Zenith, their imps were much stronger than what we fight.)