Dark TV shows aren't a problem. Walking dead has gore, the 300 movies have gore. Game of Thrones has main characters getting killed off frequently and Twitter explodes because of it every time.
A 40K movie would have to be animated but to Hollywood standards for the correct visual effects. A TV series might work but it would be big budget and at least allow plot points to be explained at the right time over 20+ episodes per season.
Plus it would be fairly difficult to have so many post humans walking about. They would need introducing one or two at a time. A few scenes showing their absolute power over the battlefield and combat before showing a company of them taking out a big target in a hugely satisfying and post apocalyptic way.