Eddie Brock is a religious fanatic with ample reasons in his deeply troubled mind to believe that he is a servant of God. Raised as a Catholic, Brock was held in contempt by his father, Carl, who silently resented his son for the death of his wife in childbirth. Brock craved his father's affection but only earned a small amount after he became a journalist for a New York newspaper, the Daily Globe. That respect was lost after Brock wrote a story that erroneously named a disturbed man as the serial killer known as the Sin-Eater. The real Sin-Eater, Stan Carter, was soon captured by the vigilante known as Spider-Man.
Brock was immediately fired, lost the respect of his father and was divorced by his wife. Despondent over the turn his life had taken, Brock went to a local church to pray for forgiveness for the suicide he was planning. While in prayer, a black, liquid like creature (identified by lead scientist Reed Richards as an alien creature called a symbiote) appeared from the darkness of the church and attached itself to Brock. The creature had tried to attach itself to Spider-Man and had nearly been killed in battle with him. Sensing that Brock's hatred of Spider-Man matched its own, the parasitic symbiote chose him as its new host. Brock accepted the "gift" of the symbiote and the powers it gave him. (Super human strength, invisibility, creating webbing, evasion of Spider-Man's early warning "spider sense" and knowledge of Spider-Man's identity.)
Calling himself Venom, Brock went on a personal crusade to kill Peter Parker. In his mind, Spider-Man had destroyed Brock's innocence. He clashed with Spider-Man and Peter Parker numerous times, threatening his friends and loved ones at the same time. He and Spider-Man only manage to call a truce (albeit a temporary one) when Brock's ex-wife, Anne Weying, convinced him that killing Spider-Man would harm other innocents. As Brock is obsessed with innocence and guilt, he allowed Spider-Man to live in order to protect the innocent. (Amazing Spider-Man #375)
Years later, Brock revealed that he was dying of cancer. With his time running short, he attempted to atone for his sins by auctioning off the the symbiote to the highest criminal bidder and donating the money to charity. After the auction, Brock again attempted to commit suicide, by slitting his wrists. (Marvel Knights Spider-Man #6) While recuperating in a hospital, Brock learned that Parker's Aunt May was a patient in the same building. He quickly killed a nurse, to see if he was still strong enough (physically and mentally) to take a life. All the while, hallucinations of the symbiote mocked him, asking if he was really serious about killing an old woman. Peter finds Eddie standing over his comatose aunt's hospital bed and Brock jumps out the window. Peter saves him by catching him with his webbing. (Sensational Spider-Man vol. 2 #39)
After being cured of his cancer by Martin Li's powers, Brock was inadvertently transformed into Anti-Venom. Brock had come to hate the symbiote and all of the harm he blamed it for. He now wants to cleanse the world of all symbiotes (the creature spawned several times) while aiding the hopeless and weak. He sees being Anti-Venom as a blessing. (Amazing Spider-Man #573)
Poisoned Thinking: Obsessive Behavior
To Eddie Brock, everything wrong in his life comes from Spider-Man. His desire to see Spider-Man dead is the driving force in Brock's life. His hatred causes him to attack Spider-Man even when he is powerless to truly be a threat. While Brock was separated from the symbiote, he was confronted by Spider-Man, who accused him of threatening his wife. Brock screamed "It's all your fault! Every bad thing that has ever happened to me...it's all because of you!" (Peter Parker: Spider-Man vol. 2 #9) Spider-Man easily held off Brock, who all the while insisted that he wanted his enemy to die. Brock was soon after reunited with the symbiote and they went to go see Brock ex-wife, Anne. After several adventures with Venom and Spider-Man, Anne was suffering from extreme stress and anxiety. She committed suicide by jumping out of her apartment window, as a fight between Venom and Spider-Man raged outside. Venom blamed Spider-Man for her death. (Amazing Spider-Man vol 2 #19)
A Holy Cause?
Religion is a major factor in Brock's life. While being very religious is not a sign of mental illness, the way that Brock injects religion into his grudge against Spider-Man and his transformation into Venom does show delusional thinking. Brock also has animosity towards the Sin-Eater (and his copy cat), the serial killer that he misreported on, causing him to lose his job. The name "Sin-Eater" may also give Brock reason to believe that religion is a part of his struggles in life. After escaping from The Vault, Brock hid at an abandoned fallout shelter under New York, There, he ranted about he and the symbiote were now "safe to pursue our destiny! A destiny begun when Spider-Man destroyed my life!" (Amazing Spider-Man #316)
Even in their very first meeting, Brock was dressed in a priest's robe and collar, claiming that the clothing was appropriate as they were "about to exorcise a demon!" (Amazing Spider-Man #300) Shortly before admitting to having cancer, Brock has visited a confessional and told the priest "I think the Lord decided to punish me for my terrible sin. I think he put a demon inside me." (Spectacular Spider-Man vol. 2 #1) Before auctioning the symbiote, he claims that he experienced a religious renewal after seeing a recent religious movie, insinuated to be Mel Gibson's "The Passion of the Christ." (Marvel Knights Spider-Man #6)
Psychopathology: Paranoid Schizophrenia
It seems appropriate that schizophrenia means "split mind" as there are two minds that make up Venom: Eddie Brock and the symbiote. While it has been shown that the symbiote goads its host into acts of violence in order to feed off of the human's adrenaline, Brock is capable of violence without the creature's influence. He also shows paranoid, delusional thoughts. He repeatedly blames Spider-Man for problems he had before he even met the vigilante. He has suffered from visual and auditory hallucinations (Sensational Spider-Man vol 2, #39) and has thought of or attempted suicide on at least three separate occasions.
Schizophrenics often prefer to be left alone and are non-violent. They harm themselves much more often than they hurt others. This seems to be the case with Brock, who went to the church alone to pray for forgiveness for his upcoming suicide. It was only after being bonded to the alien that he showed signs of aggression. As the creature provokes violence, his behavior may have been permanently changed by the alien. The multiple suicide attempts do fit the schizophrenia diagnosis. Schizophrenics are fifty times more likely to die by their own hand than someone from the general population. Within ten years of a schizophrenia diagnosis, 10% of individuals have died by suicide. Its been found that those born in an urban area are 50% more likely to develop the disease. (Brock was born in San Francisco.)
If Brock can be successfully and finally separated from the symbiote and with a strict regimen of anti-psychotics, he may stand a chance of some degree of recovery. While total recovery is rare (25% of patients are cured after ten years of treatment), one quarter of patients are "much improved, relatively independent." With therapy, he may be able to see how distorted his obsession on Spider-Man is. This will be difficult, as the symbiote has shown to be fixated on Brock and is very hard to kill.