If Deadpool killed the Marvel universe, could he kill the whole DC too?
No and he didn't kill the Marvel Universe it was an alternate universe story set in Earth-12101 universe, which is the only place it is canon.
It is not canon in the 616 universe which is the prime Marvel Universe. I am a big fan of Deadpool but I am not a fan of these alternate universe stories that defy "Superhero logic" if there is such a thing. Usually in those alternate universe there is no consistancy and the villains seem over powered and the heroes are there for plot placement to advance the story of the protaganist and the usual rules don't seem to apply like when King Hyperion was said to have killed Galactus from his own Universe and the likes of all the heroes were unabe to stop him.
Well Deadpool didn't kill the Marvel Universe. He couldn't kill Kitty Pryde or Wolverine. A number of deaths were off panel because there's no conceivable way he could beat the likes of Thanos, for example no matter how much prep time he had. He didn't kill Taskmaster (and to my recollection, Man-Thing), and if I recall correctly, beings like the Asgardians also weren't killed (with the exception of Thor).
In addition, a number of characters had their powers taken away (a close-range gunshot would still trigger Spider-Sense, Bruce Banner's transformation into the Hulk is commonly accepted to happen any time their life is threatened (even the very second the sword hit his neck). So the first part of that is already false.
Could Deadpool kill the DC universe? Still no. Even armed entirely with Kryptonite, there are still 150 ways Superman could beat him, and a lot of them don't include attempting to kill him. He could never tag the Flash. In general, he's severely outmatched by almost the entire Justice League, let alone characters like the New Gods, Black Adam or the Reverse Flash.
Furthermore, the Green Lantern Corps (or really any Lanterns) would obliterate him in less than seconds. Any Kryptonians that don't attribute the same... value to human life that Superman does would also take him out so quickly he wouldn't even have time to realise that he was in the DC universe to kill people.
Tl:dr He didn't kill the Marvel Universe and there's no way he could take on the DCU either.
Sure, why not? Deadpool killed all of the heroes and villains in a Marvel universe where those heroes and villains were significantly weaker than usual, so there's no reason he couldn't kill a DC universe where everyone was weaker than usual (aside from licensing reasons).
Of course, it bears repeating that he was only able to win because it was an alternate universe which was specifically written so that he would be able to win. Anyone could win, in those circumstances.
The only one (short of The-One-Above-All) who could possibly defeat every hero and villain in the main Marvel universe (Earth-616) is Squirrel Girl, which she did in her own graphic novel, "Squirrel Girl Beats Up the Marvel Universe"; and which is canon.
Any character could potentially kill every other character.
That's why if they ever do more DC/ Marvel crossover comics, I'd love to see them take a page from Deadpool Kills the Marvel Universe, Punisher Kills the Marvel Universe, Deadpool Kills the Marvel Universe Again, etc. and have a series of books which explores how 1 character could take out or take down the entire other universe full of characters single handed.
Batman Takes Down the Marvel Universe
Superman Breaks the Marvel Universe
Flash Outpaces the Marvel Universe
Joker Pranks the Marvel Universe
Deathstroke Kills the Marvel Universe
Spider-man Webs the DC Universe
Wolverine Fillets the DC Universe
Blackbolts Has Words for the DC Universe
Doctor Doom Invades the DC Universe
Galactus Eats the DC Universe
Definitely. It would likely be by chance, accident, or simply outliving then in a disaster he created. Lobo can kill the DCU so Deadpool definitely could.
Well,the times he killed the universe were pretty writer based,by his own abilities he couldn't kill any entire universe,but if you give him an author that can identify some breaches,yeah,sure.
Tip:If someone actually did this,it'd probably be related to magic of some sorts