Debunking time! This is actually a rumor. Deadpool didn’t kill the entire Marvel Universe any more than Superman turned evil, or Batman killed Joker.
The storyline where Deadpool infamously killed the Marvel Universe occurred outside of main comics canon. It was just a fun elseworlds storyline, of what might happen if Deadpool tried killing the Marvel Universe. The Deadpool kills the marvel universe was an off-canon, one-shot story and it was purely meant for fun.
In fact, it technically wasn’t even fully that… “Deadpool Kills the Marvel Universe” was really just another cash grab, to capitalize on the success of the storyline “Punisher Kills the Marvel Universe” (nobody ever talks about this story anymore, though).
“Deadpool Kills the Marvel Universe” is an interesting read, I suppose— especially if you like Deadpool. The story consists of Deadpool being driven insane by Pscho-Man, and going on a killing spree. But, let’s be real: Deadpool has no shot at killing the entire Marvel Universe.