It's the first mini-series in the Deadpool Killogy, which includes:
1. DEADPOOL KILLS THE MARVEL UNIVERSE (2012, 4 issues) = Psycho Man tries to turn Deadpool into a slave warrior and instead unwittingly persuades him to murder all the costumed heroes in all the universes. Instead of being a joke character who breaks the fourth wall, this version appears to have started taking seriously what it means to be a killer in a superhero comic book universe. He doesn't like it. This version of Deadpool has since been nicknamed "Dreadpool".
2. DEADPOOL: KILLUSTRATED (2013, 4 issues) = Dreadpool tries to kill fiction itself, i.e. he goes hunting characters from classic literature.
3. DEADPOOL KILLS DEADPOOL (2013, 4 issues) = Dreadpool goes around killing alt.universe versions of himself, e.g. Deadpool Pulp, Lady Deadpool, Kidpool, Dogpool, and Headpool.
4. DEADPOOL KILLS THE MARVEL UNIVERSE AGAIN (2017, 5 issues) = guess what.
(Garth Ennis got there first in Punisher Kills the Marvel Universe, of course. This book pays passing homage, since Deadpool knows his comics history.)
I bought this book on a whim, because I was buying something else from the same Ebay seller... but it's quite good. Deadpool's observations aren't as flippant as you'd expect. This would quite well as a companion piece with certain much more heavyweight Alan Moore stories, for instance, with the twist that Deadpool's not just killing "names have been changed for legal reasons". He murders the real Marvel superheroes (and supervillains), in some cases twice because he's not the only one with a healing factor. He shoots Spiderman in the head. He decapitates the Watcher.
Obviously, it's too easy. Even with Deadpool's ability to regrow his body even after becoming pavement pizza, he shouldn't have been able to kill all these super-strong people. As with Punisher Kills the Marvel Universe, the death parade isn't entirely convincing, but is necessary for the book's concept. (One or two of the deaths are memorable, though. I liked Mr Fantastic's melting demise and our brief glimpse of Howard the Duck.)
We also briefly meet Man-Thing. I'd never encountered the character before, so I was a bit surprised. He's a swamp monster who lives in the Florida Everglades and his character design reminded me a lot of DC's Swamp Thing, another swamp monster (but that one's in Louisiana). Also, both characters first appeared in 1971, but Man-Thing came out first by two months.
This book works on two levels. The obvious level is as per the title. Lots of kills, many of which happen gruesomely to Deadpool before he gets back up and does even worse to his opponent. This is fun. It's quite common for famous characters to die like flies in Elseworlds and this is just another take on that. The other level, though, is Deadpool's dark opinions about the nature of comics themselves and why it's surprisingly rational to go around killing everyone to save them from the sadistic universe they're living in and from, paradoxically, themselves.
"Your tendency to come back from the brink of death has nothing to do with your healing factor. Your mutant factor isn't regeneration. It's popularity."