Hellblazer/Nightmare Fuel


 * In the very first issue of Hellblazer, John Constantine drops by to visit his friend Gary "Gaz" Lester. Letting himself in, he finds that the place has an awful stench and hasn't been cleaned in weeks. The syringe he finds with tiny bug carcasses suspended in fluid is disturbing, but when he enters the bathroom and finds Gaz sitting in the bathtub complaining: "... it feels like there's bugs all over me", it's pure nightmare fuel.
 * What's causing Gaz's problems is even worse: he's become the host of a famine demon who causes people to literally consume the objects of their desire, i.e. start gobbling up money or jewelry as if it's the last, most delicious food on earth (see the liquor store scene in that film with a similar premise)-- one disturbing panel shows a bodybuilder chewing on his own muscular arm. The only way to keep innocent bystanders safe is for Constantine to seal Gaz alive in a wall.
 * There's also Rare Cuts, a compilation of the most High-Octane Nightmare Fuel-o-riffic Hellblazer short stories, which culminates with John and a friend trying to save a village from a mind-warping pagan festival... with masks involved. Those big, hideous, over-the-whole-head ones. And guess who's the only survivor.
 * How about the Son of Man arc? And then the story gets really nasty.
 * Also the flashback when he's begging to be allowed back into Ravenscar, even though they torture him on a nightly basis in there.
 * The Horrorist, where John has stopped even caring about the awful things he sees.
 * Hard Time--what he does to the prison.
 * Highwater, guest-starring some of the crowd from Preacher. (Not really, but they might as well be.)
 * Or more recently the Scab storyline, where John is Euuuugh.
 * The Newcastle Incident itself. It's not what happened that's so bad (we've seen worse since), but what's haunting about it is that it all happened because of recklessness and ignorance.