Deadpool Kills the Marvel Universe #3 p8 (Deadpool Kills X-Men Pt2)

The first panel shows Deadpool killing Colossus, Gambit and Rogue. The remaining panels show Deadpool baiting Prof X into reading his mind.

Once Prof Xavier does that, he realizes that Deadpool has seen beyond the fourth wall. He then comes to the realization that he, all his X-men, and Marvel Universe are just comic book characters.

This blows Prof X's mind leaving him brain dead.

Turns out Madcap was right, and him merging with Deadpool so long ago was the impetus of Deadpool's insidious inner voice. After Prof X had Deadpool committed, it was Psychoman's probing that clarified the existence of the 4th wall.