The initial procedure that causes Deadpool to descend into his rampage. In particular, seeing that whatever it's doing is causing Deadpool and both inner voices to react in complete and utter agony, with a new, monstrous voice emerging and claiming to have eaten the other two. It becomes an even bigger Fridge Horror when Deadpool's main book eventually revealed that the white text box voice was an actual person (Madcap) .
Dreadpool's monstrous rampage has many scenes worthy of this, such as when Sue Storm witnesses Dreadpool decapitating The Human Torch and is caught by the assassin when Johnny's blood splatters on her.
There's also the final panel of the comic where Dreadpool looks directly at the reader and says he'll come for them next. His "shush" gesture doesn't help.
Lets just say Dreadpool's murderous rampage in general.
The entire premise of Again falls into this territory: Deadpool has been brainwashed by an alliance of villains led by the Red Skull, and they're using him to murder every single one of the heroes. It gets even more nightmarish when it turns out that part of Deadpool's mind is aware of what's happening, but is completely helpless to stop himself from committing any further atrocities.
Again also gives us the deaths of Ms. Marvel, Power Pack, and Moon Girl. Deadpool hallucinates the entire encounter as a cartoonish game of hide-and-seek... before we cut back to reality, where we see a horrified crowd staring at whatever it was Deadpool did to them all.
The cover to the first issue of the first series, featuring Deadpool holding the decapitated head of the Hulk, much to the rightful horror of Captain America, Spider-Man, Storm, and Iron Man.