A Warhammer 40,000 space marine is not just your usual sci-fi super soldier. They are hulking behemoths, 10 feet tall (and about 10 feet wide too). They don’t move like people, they move like freight trains. They have the physical bearing of an industrial freezer unit and the emotional intelligence of a crocodile. They do not retreat and they do not surrender, and their 9-to-5 is fighting the worst things you can possibly imagine over and over in a completely futile forever-war.
but humanity is evil too
Emphasis on this. We humans have become Xenophobic Christofascists* turned up to 11. All aliens are bad** and anything against established doctrine is heresy of the highest order. Human labour is essentially free vs the gross expense of materiel so the leadership will think nothing of having entire generations of a planet mine out some toxic substance that kills before you age much past the ability to outbreed it.
In short, anyone who claims humans are the good guys, is misguided at best.
*EmperorFascists as the ruler is the Immortal God Emperor.
** Officially, but there exists means and people who can deal a little more diplomatically than with a gun.
