With Batman's rogues' gallery containing all the most gross and grotty villains, as well as some of the most genuinely unhinged, you have all the necessary ingredients for some well and truly dark moments in the character's comics.
While the Joker alone could provide an entire menu of depraved and disgusting storylines, he's often overplayed as the sole producer of messed up events.
There's a good chance that his popularity means that many of his worst moments would see the big screen anyways, as we've seen with the animated version of The Killing Joke. But there are still many other various vagabonds who can be held responsible for a whole chocolate assortment of disgust. Everything from the torture of children to Batman himself being made into a fleshy human door features in the weird and wonderful pages of DC's most brooding hero, and it all manages to happen while still feeling feasible within the sordid streets of Gotham. (Well, unless you count All-Star Batman and Robin, of course.)
While the Dark Knight isn't always DC's darkest hero, he certainly has cornered the market on having endless content that would never make it to the big screen...