They tell the story of the space marines as the good guys.

Also neurodivergents can probably get around the hero to root for limitation by just enjoying the cooler looking faction being their guys. So coolness as a replacement for good.

Even Hitler honestly believed he was the hero of his own Star Wars.

Once everyone is a bad guy who cares about the conflict or death? The world is and will be shit no matter the resolution. The normal response to a grim setting with no hope: . It starts to becomes logical to kill everyone, nuke the entire setting, and restart from ash because the post apocalypse seems preferable.

I prefer the Imperium in a constant state of civil war. Where imperial governors have inquisitors killed for complaining about his alien subjects and no one notices because communication tames centuries . Where where the Reason for the shitty technology is because the admech are hoarding the good stuff and they secretly have laboratories, robots, ai, and loading mechanisms but you can't have any because you're a stupid meat creature and the omnissiah will devour you along with all the other weak flesh creatures. Where imperial guard regiments are made up of xenos because some bureaucrat thought the form for human worlds was easier to fill out and going against the bureaucracy gets you killed. Where space marine chapters blow up imperial navy ships and rogue traders for kidnapping from worlds under their protection. The imperium should be just another evil for the heroes ro struggle against.

The Tau is less evil because the options are "submit or die" when the imperium usually just gives you the option of "die". They need more little alien empire factions. Including one who uses Imperium tech but knows hoe it actually works. Some can be idealistically peace lovers whose army consist of five guys who can devastate entire legions, pack predators, alien lizardmen who declared holy war on humanity for its many crimes, the ragtag survivors of species the imperium genocided, borg ripoffs, pinky and the brain style ratmen who rule over a subdued population of humans and view the emperor as a nascent chaos god, cute little teddybear creatures who speak of friendship and love as they tear the guts out of a screaming spacemarine, dalek ripoffs, borg-cybermen ripoffs, proper exodite models, surviving iron men, and predator ripoffs themed after fox hunters and modern big trophy hunters.