As always, Cain tends to downplay the situation in his usual manner.

I second reading or listening to the Ciaphas Cain books, since they bring that pinch of satire with them.

You also get to explore most races from the 40k universe quite quickly.

I think reading the opening text on any book fully gives you a good grasp. Often it’s abbreviated to „in the grim darkness of the far future, there is only war“ All the other lines explain quite well, why humanity would be so inhumane in this scenario and why there can be only war. Basically all factions but especially humans live in different flavors of fascism in 40k. Orks might be a bit of an outlier here. Warhammer is a fun tabletop game and has awesome pc games and books. The lore is (or used to be) very critical and satirical on where humankind might end up in a science denying, fascist dystopia (basically the opposite of Star Trek)

Despite all criticism and satire, Warhammer itself is difficult on misogyny (e.g. only men can be space marines ™ because of genes), toxic masculinity & warrior culture, queer (with some positive exceptions) and trans topics (again, genes and also demons get mixed up here). Sadly the lore wise intergalactic xenophobia and racism also seems to be an open door to neo-nazis who turn the hobby into their racist fantasy. (Something I didn’t grasp but quickly realized looking at some armies of other players on tabletop and them talking).