Every super villain had a screwed up childhood but Cletus Kasady's was extra crazy. He loves to taunt his doctors with whether or not he was abused or if he is just a bad seed. It may be a bit of both but genetics alone don't make up an individual's personality. What is suspected is that he watched his father beat his mother to death with a hammer. After his father went away to prison, Cletus ended up with his grandmother. She found him torturing her dog and when she tried to stop him, he killed her. The police took her death as an accident and Kasady ended up at St Estes Home for Boys, in Brooklyn. The building was later destroyed in a fire.
Over the years, Kasady became a serial killer and was sentenced to eleven consecutive life terms in Riker's Island prison. While incarcerated, Cletus was cell mate to Eddie Brock, aka Venom. Eddie was without the symbiote at the time, thinking the creature dead. It proved him wrong, helping him break out of prison. At that moment, the asexual creature spawned, leaving a new symbiote with Kasady. The symbiotes have no concept of family and it felt no obligation to the spawn or the need to even tell Brock it had reproduced. Now bonded to a new alien, Kasady escaped and began a murder spree as Carnage.
A Nihilistic Philosophy
When a new therapist was sent to Ravencroft Asylum, Kasady made his philosophy known to him during one of his stints of incarceration. As the symbiote tore into Dr. Kurtz, Kasady raved "I'm chaos, Mistuh Kurtz, chaos--and the rest of the world wont admit that its just like me. And because of that the world is worthless, its dirt on my heel--its a planet full of mannequins filled with blood and they may as well be torn up and burst and stamped on because what else are they good for--apart from making noise and being nice and making more stupid little mannequins that sit still for lies and TV and kissy-kissy garbage." (Carnage: Mind Bomb)
According to the Internet Encyclopedia of Philosophy, "Nihilism is the belief that all values are baseless and that nothing can be known or communicated. It is often associated with extreme pessimism and a radical skepticism that condemns existence. A true nihilist would believe in nothing, have no loyalties, and no purpose other than, perhaps, an impulse to destroy." Indeed, Carnage has shown loyalty to no one. He has attempted to kill Doppelganger, a member of his "family" that tore apart New York (Spider-Man vol. 1 #37) and Bill Bentine, a childhood friend (Amazing Spider-Man (Vol. 1) Annual #28). He also fantasized about murdering Shriek, his lover from the "Maximum Carnage" riot. (Carnage #3)
Mass murderer, Spree killer or Serial Killer?
The FBI divides murderers into three different categories, each with different sets of motivations. According to former FBI agent and criminal profiler John Douglas, this is how various killers are classified:
A serial killer is someone that has murdered on at least one occasion, with an emotional "cooling off" in between kills. A cooling off period can go on for days, weeks, months or years. Rarely, its only for hours but each murder is always a separate emotional experience for the killer.
A mass murderer kills four or more people in one place in one incident. The killings are all part of the same emotional experience and the incident may go on for minutes or hours.
A spree killer is a person that kills at two or more separate locations with no emotional cooling off period. The killings tend to take place over a short period of time.
Cletus Kasady: Mass Murderer
Kasady has never been one to hold back on his rampages, especially the event called "Maximum Carnage" where he and a gang of other murderers wrecked havoc through Manhattan for days. Of course, its unusual for a murder rampage to go on for that long, but the symbiote does increase aggressive tendencies by provoking its host to violence and feeding on adrenaline. (Being given super human strength, stamina, agility and being bullet proof could certainly help extend a rampage as well.) Given Kasady's disregard for planning and thought (he once told Shriek that thinking was a nasty habit that she better stop doing), he likely never put any thought into how it might end. This is a trait more common in spree killers than mass murderers, but again, possibly a side effect of the symbiote's influence.
Psychopathology: Mania and Narcissism
Carnage has very grandiose ideas for a simple mass murderer. He has a vision that his creed of freedom via chaos will take the world by storm. He is also in love with murder, saying that it makes him feel like a "God on Monday morning" or "like a gunship." (Carnage: Mind Bomb) He also considers the world to be "dirt on my heel" because it has so far rejected his philosophy. These traits are commons in manic and narcissistic patients, respectively. Again, the effects of the symbiote on Kasady's psyche is uncertain, but the power and adrenaline could definitely add to the grandiosity.
Kasady's exact background is uncertain but some form of abuse is likely. An aggressive individual in his own right, his condition could only be exacerbated by the alien's influence. A highly emotional person to begin with, the "cooling down time" of a typical mass murderer seems to have been negated by the adrenaline feeding parasite. Separating Kasady from the alien is of greatest importance but has proven to be difficult to accomplish. Even after successful separations, the creature finds a way back to its host.