I think that seeing Warhammer 40K as dark is kind of missing the point. Sure, looking at the universe as a real place is horrific. There's few worse places you could exist, almost no where that contains objectively more suffering. But the point to Warhammer is not to take it seriously. It's to create a world so absolutely devoid of the good and pure mucking around in morally grey areas, that you can focus entirely on just how cool all the horrible things are. If you had Tyranids fighting the rebels from Star Wars, then you would be emotionally invested. You would want to see the characters you like succeed, you would hate the villains, and you would hope that pluck, determination, and a morally upstanding philosophy can triumph over adversity. But when Tyranids fight Space Marines, well, you have no moral investment, and you can observe glorious, stupid violence without any nagging moral considerations. And when undead space robots, super bugs, quasi steampunk religious fanatics, maurading green monsters and insane cultists with chainsaws all go for each others throats with no need to consider any moral justification because EVERYONES an asshole, you can enjoy the pure, unadulterated carnage. Because no matter what happens, you don't have to worry about the good guy losing getting in the way of enjoying the chaos, and that is surprisingly cathartic. maybe only good in small doses, but still.
Basically, Warhammar operates on the same principle that made the EA/Zynga court debacle so fun for us.
