Carnage was once a serial killer known as Cletus Kasady, and became Carnage after merging with the offspring of the alien symbiote called Venom during a prison breakout. The symbiote amplified his psychotic nature making him even less mentally stable than he had been previously, and therefore even more dangerous. An interesting note is that, unlike Venom, Carnage is a singular entity, referring to himself as "I" instead of "We", because the symbiote has actually merged into Kasady's bloodstream. Carnage is also the "father" of Toxin. A note on his childhood: Cletus' father killed his wife (Cletus' mother) because she tried to kill Cletus. Cletus then testified in court against him, saying that he killed her for no reason, causing him to die by electric chair as means of punishment. It is also unclear on Cletus' relationship with his mother, as he seems to have feelings for her, causing him to dig up her grave at the end of his massacre through New York. He also revealed that he killed his grandmother when he was younger, pushing her down a flight of stairs. Since the alien symbiote adapts to the host's personality, the Carnage symbiote is affected by Kasady's insane mind and lust for destruction.
Kasady has been separated and re-attached to the symbiote multiple times. The Carnage symbiote has occasionally attached itself to other hosts, including the Silver Surfer, Ben Reilly (a clone of Spider-Man), and Doctor Octopus. Venom later decided to absorb the Carnage symbiote "for good", during which Kasady retained the Carnage persona by costuming himself in red paint and continuing his killing sprees (albeit as a powerless human). This was short lived, however, as Kasady coincidentally found an exact replica of the symbiote in the Negative Zone. As to whether or not the symbiote is a separate entity, "permanently" bonded, or his own blood mutated, has not been revealed. The symbiote once chose to leave Kasady and bond with Ben Reilly. Kasady, without the symbiote, rapidly aged and his body quickly deteriorated. In this state, Kasady was barely able to function, having a hard time operating a sink, let alone being a threat. The symbiote bonded with him again in time to save his life, regenerating his health. When the symbiote abandoned him yet again, this time to join with the Silver Surfer (transforming him into the Carnage Cosmic), Kasady was left dying from an accelerated form of stomach cancer (apparently a side-effect of the symbiosis, also suffered by Eddie Brock, the original Venom). However, when Venom ate the symbiote, he did not deteriorate, indicating that perhaps some fragments of the symbiote were left behind (enough to sustain him but not enough to form the costume). This may be backed up by the fact that Kasady seemed to retain at least some enhanced strength.
Later, Venom and Spider-Man had to team up to stop Carnage. During this time it was revealed that the symbiote had become a part of Kasady's blood and was irremovable from him. Kasady only needed to be cut for the symbiote to flood out to become Carnage once again. Unlike Venom, Carnage had become less vulnerable to the high-pitched sounds which would normally stop Venom, however he was still vulnerable to intense heat.
Recently, Carnage was one of many super-villains trying to escape from the Raft. He tried to kill couple helpless civilians who were there to meet another inmate called Sentry. Who then intervened and flew Carnage outside of the Earth's atmosphere, where he ripped him apart. Kasady was presumably killed and has not been seen since.