I'm not too knowledgable on all the lore, but iirc:

• Space Marines are engineered supersoldiers who see humans as puny livestock
• The leader is an emperor, also known as god-emperor, whose word is law
• Pretty much all of their technological advancements exist to kill/control
• Their creed is basically to eradicate all non-believers (heretics) and non-humans (unless the non-humans can benefit them)
• Slavery is pretty much the standard for punishing people who oppose the god-emperor (if not death)
• A lot of other things

Basically, the entire main faction is a hyper-fascist, fundamentalist, imperial, and ultra-violent organization. A lot of the other factions operate with a similar amount of cruelty and violence.

The point is that no faction in Warhammer can really be known as "the good guys." I always perceived the 40k universe as a caricature of some of the worst aspects of mankind, but I play along as a cog in the machine, knowing it's bad. Neo-nazis tend to perceive the Astartes/Space Marines as glorious heroes. It's akin to alt-righters enjoy Starship Troopers and Rage Against the Machine, without recognizing the irony.