Massacre was an alien of unknown origin who traveled the galaxy, killing anyone he felt like just for fun. Occasionally he would take a hit for payment, but for the most part he seemed concerned with just going around killing for sport. The one exception he made was to an alien known as the Skimmer, who would tail Massacre and scavenge through the belongings of Massacre's victims. Massacre is known to have killed thousands of people across hundreds of worlds. He fought and killed Auron on a distant, alien world. Auron had tracked him down from an earlier encounter with help from Superman. After a brief fight with Superman, Massacre withdrew, promising to hunt him down soon.
Massacre eventually came to Earth, arriving after the Fall of Metropolis. He first arrived in Metropolis, IL but soon found the right city with his prey. Massacre toyed with Superman, beating him mercilessly while avoiding any hits from the Man of Steel. Superman soon realized Massacre could sense his nerve impulses and managed to overcome that handicap, putting a major pounding on Massacre. During an attempt to help Massacre, the Skimmer was fatally injured. Massacre quit fighting, taking the body of the Skimmer away. While he could have gotten him aid, Massacre let him die instead, delighting in thinking that the fight between he and Superman would now be sweeter as the Man of Steel was his enemy, not just his prey.
Massacre never got the chance, however, as he was captured by the Tribunal, who managed to negate his powers (even his super strength) and put him on trial for genocide. Having been found guilty, Massacre was beaten to death by some of the families of his victims.
Much later, Massacre would turn up alive and well and in the service to Maxima when she was fleeing before Imperiex. He briefly fought Green Lantern while Superman tangled with Maxima. When Superman mentions he saw Massacre die, Maxima responds "And I was at YOUR funeral..." She also mentioned that Massacre served only her then, which might imply he was reanimated and/or a clone of some sort.
Massacre was still with Maxima when she led her forces to earth to fight against Imperiex and apparently perished (along with her) in the battle against the Destroyer of Galaxies.