The Birth: Another villain that Spidey takes credit for creating is Carnage. It started with the alien symbiote costume that Spidey brought back from the first Secret War. After the symbiote was rejected by Spidey, it bonded with Eddie Brick; creating "Venom". When Eddie was in jail, he shared his cell with the psycho, Cletus Kasady. Cletus was already serving 11 consecutive life terms in prison. When the symbiote busted Eddie out from the can, it left behind a "spawn", which combined with Cletus Casady; producing an even more dangerous villain by the name of Carnage.
.... Cletus had a cameo in Amazing Spider-Man #344 and made his actual debut in Amazing Spider-Man #345. But he stole the show as Carnie in Amazing Spider-Man #361 after a Carnage cameo in Amazing Spider-Man #360.

The Alter Ego: A seriously deranged mind, Cletus apparently inherited the murderous gene from his father who had killed Cletus' mother. Cletus was seriously abused in the orphanage that he grew up in; but whether he got there in the first place by killing his grandmother is a fact that's not quite clear. He decided to repay society for his misfortune by creating chaos. His philosophy was that if you have the guts, you can do anything, including murder.
Carnage's consort Shriek loves him unconditionally, even if he physically abuses her, as long as he lets her kill somebody every now and again.

The Legend: Once Cletus bonded with the "spawn" of the alien symbiote, he escaped and started spreading his message of chaos on a larger scale than ever; going on a murder spree.
Of course, he encountered Spider-Man; but as he proved tougher than the web-slinger, Spidey enlisted the services of The Fantastic Four and Venom. They were able to defeat the creature and cart his sorry butt to Ravencroft asylum.
Carnage escaped from there a couple of times and was easily able to get the attention of Spidey and Venom. While Spidey blames himself for the creation of Carnage, Venom despises his "spawn" since he stands for all that Venom opposes. Like Venom, Carnage's dual identity is publically known and he has a long criminal record as both Cletus and Carnage.
The symbiont, which is merged with Kasady's bloodstream and thus inseparable from Kasady, is capable of morphing into whatever Cletus pleases- and being the kind of guy he is, he is usually animating his costume to form weapons with sharp pointed edges to destroy his victims. as well as swing lines and snare traps. Though it's possible, Carnage does not use his costume as a camoflage, the Venom does. Carnage also posses wall-cralwing abilities similar to Venom's. And the costume; being alien in origin is able to dampen Spidey's Spider Sense and Venom symbiote's in-born ability to track its spawn. Carnage has proved considerably stronger than Spider-Man as well as Venom; probably due to it's incubation on Earth.
But like it's parent, Carnage's costume is vunerable to sonic attacks and heart based attacks; though to a lesser degree.
Carnage's known allies have been Shreik, Demogoblin and Doppelganger.

Amazing Spider-Man #361