Though Deadpool is undeniably an anti-hero, he's far from amoral. Marvel's famous Merc With a Mouth has his own sense of morality and nobility that typically makes him anything but the most brutal vigilante. Despite this, there are a few universes where Deadpool kills every single Marvel hero, and one leads to a series of murders so disgusting and disturbing that it even shook Wade himself.
Coming from writer Cullen Bunn and artist Dalibor Talajic, Deadpool Kills the Marvel Universe Again sees Wade Wilson being brainwashed by a cabal of villains. As a result, while Wade sees himself hanging out and messing around with his friends, the reality is that he's slaughtering Marvel's greatest heroes. One of the first heroes he takes out is Spider-Man. In his head, Wade and Peter are entering an eating contest, while readers see the reality is that he teamed up with Venom to devour Spider-Man. Eventually, Deadpool even wears Ultron like an Iron Man suit and completely annihilates the Avengers. Despite the brainwashing, a part of Deadpool knows what he's doing. Yet still, his grim work must continue. After tearing Ultron off his skin, Wade wanders off to commit his darkest act of all time. He goes to kill Marvel's youngest heroes.
In one of the most disturbing scenes ever put in a Marvel comic, readers see a cartoon version of Power Pack, Ms. Marvel, and Moon Girl playing a game of hide and seek with Deadpool. Though the young heroes all try to hide from Deadpool, he finds each and every one of them. In the cartoon world he tags, but in reality, he murders them all in cold blood. Eventually, with the worst of it over, readers finally see Deadpool in the real world. Surrounded by horrified bystanders and with only a small glimpse of the felled Devil Dinosaur, Deadpool remarks to himself that he's glad he didn't see that one.
Worse than just outright showing Deadpool kill these young heroes, the comic is all the more horrifying by filtering it through a cartoony world. By leaving it up to the reader's imagination and implying the sheer terror that these children are feeling, the horror is so much worse than if it had just been shown literally. Adding to all of this is the knowledge that, deep down inside, Deadpool knows what he's doing and knows that this cartoon world is false, yet he is completely powerless to stop it. When Deadpool says "In this case... I'm glad... glad I don't know," it's one of the most heartbreaking things Deadpool's ever said.
Deadpool has often been dismissed as nothing but a meme, and there are certainly times where that description fits. What Deadpool Kills the Marvel Universe Again shows though is that there is a heart to the character. He might be an unhinged mercenary, but underneath all of his jokes and meta-humor, he does genuinely care about the other Marvel heroes and it absolutely destroys him to know that he hurt so many innocent people. Deadpool's most heinous act will never be seen by fans, and that is probably the way he prefers it.