At some point in the past, Gregory Terraerton was seemingly killed by "the merc with the mouth" Deadpool.
Years later, he somehow came back to life as Slayback and used stolen Weapon X files on Kane and Deadpool to kidnap Copycat in order to use her as bait to find and kill Deadpool for killing him earlier.
He was subsequently seemingly killed by Zero (an A.D.A.M. unit).
Slayback once hired himself out to a scientist group who wanted to use Deadpool's healing factor as a possible cure for the Legacy Virus. He enjoyed carving on his old friend Wade's chest during the experiments.
He was later revealed to be alive, but he was killed by Sharp after test subjects from the Weapon X program escaped.