Warhammer 40,000 is known for its harsh, unforgiving universe. It is a galaxy where everything is lethal to the point where just surviving is a major victory. The worst tends to get brought out in everyone, and even the best of intentions become warped and twisted into doing more harm than good. The 40k universe can get so dark, it makes other sci-fi worlds look almost utopian by comparison. You know things are bad when Alien looks like a vastly better universe to live in. Between constant warfare, chaos gods, hostile environments, and the seemingly endless swarm of tyranids pouring in from another galaxy, chances of survival are pretty low. Even the major dominating species in the galaxy such as humans, aeldari, and orks are constantly struggling to keep themselves afloat. It's hard to imagine anyone having much of a fighting chance.