Carnage is the offspring of the alien symbiote Venom. It bonded with the crazed serial killer, Cletus Kasady, thus creating one of Spider-Man's most murderous, brutal, and lethal adversaries.Origin
When the Venom symbiote returned to bond with an imprisoned Eddie Brock again, it unwittingly left behind the symbiote's offspring (The symbiotes reproduce asexually, once per generation. The children are then of no consequence to the parent. Thus, the Venom symbiote felt no need to inform Brock of the existence of the new symbiote). This Venom offspring got into a cut on Brock's cellmate Cletus Kasady, an insane serial killer, thus bonding with his blood and transforming him into Carnage (also causing his symbiote to turn red). He escaped prison, and began a series of murders, and at the scene of each crime, he wrote "Carnage Rules" on the walls with his own blood.
Carnage was created by David Michelinie and Mark Bagley.
Carnage and Cletus were always like minded; they both share a love of killing. Cletus Kasady is in complete control of the symbiote, unlike Eddie Brock who still puts up some fights with his own alien. Unlike Venom, Carnage refers to himself as "I" instead of "we" because Kasady and Carnage have become one being literally to the point that the symbiote lives in his blood. Perhaps due in part to his insanity, Carnage's strength and abilities far exceed those of his predecessors Venom and Spider-Man, and grows with his blood-lust.
Carnage was actually not the first choice of names from his creators, as revealed in the trade paper-back Spider-Man: Carnage, his original name was Chaos, but due to a DC character sharing that name, it was rejected. The 2nd choice was Ravage, but due to concerns of him being tied to Ravage 2099 that name was also rejected, Carnage was actually the third choice when it came to naming the character.