Commissar Sae said:
It really depends on what side you look at. I can laugh it up with the best of them but reading through some sections of fulgrim or Nemesis were actually kind of fucked up. Watching a stage show where the lead dancer starts literally breaking herself apart as she gets more and more possessed by the music or having a creature basically be an even more messed up version of 'the thing' has a certain power to it.
At the same time a lot of it is over the top and ridiculous to me. Mostly the space marine stuff honestly, since work based around the inquisition or the imperial guard can actually be pretty good.
Though I agree with what you say about the levity of the universe, I would like to stress that even in fulgrim, your not supposed to take it seriously. It is dark for sure, but it's OTT for shits and giggles, or for shock value depending .
When the renegade Emperors Children Legion start wearing the skin of their enemies you have to step back and go "yep, that's about as Evil as things get in this universe". Or that sub-plot with the remembrancer turning human waste and matter into "art" you say "sounds like Slaanesh, all right".
You Roll with the punches so to speak.
Just as people don't take Adventure Time or MLP: Friendship is Magic seriously (they really shouldn't mind you) you don't take Warhammer seriously. It can be cool, messed up, intriguing and dark and still be a entirely made for the larf (at every level). Not every dark fiction has to be 'A Song of Fire and Ice' levels of "SRS BSNS!"
That said, there are some really memorable moments in the Horus Heresy series. Still not taking it seriously, but loving it all the same.