John Doe is a psychopathic serial killer obsessed with the Seven Deadly Sins of Christianity, and he kills his victims in symbolic murders in an attempt (at least in his own mind) to remind the world about the inevitability to avoid these sins. His insanity is based on fanaticism and obsession but did little to damage his knowledge; in fact, his madness may have added to it as he is portrayed as a twisted mastermind who is comfortable with toying with the authorities. Doe particularly seems to enjoy playing games with the film's protagonists, Detective David Mills and Detective William Somerset, in a similar fashion to killers such as Jack the Ripper. John had a bad childhood which became a major factor of his crimes. He suffered headaches because of the many shock treatments he endured. As he got older, John's mom often beat him with a spoon, belt, or her fist, which would cause John's nose to bleed. John, now a grown man with a wife, found her cheating on him; John never had children because his wife was unable to so he never touched her. He had a normal job until he had a disagreement with is boss about the handling of the company that John worked for and he was then fired. John visits his mother at the same home he lived as a child and in his teen years found out his mother was burning his belongings she tells him he was going to hell for his sins. John's mom was caught on fire while she was burning John's stuff, John found a key and left while his mother burned to death. John use the key for a box that contained the book of the Seven Deadly Sins, this started his murderous crimes.
Each of Doe's murders represent one of the Deadly Sins, and include:
- Forcing an obese man to feed himself to death (representing the first sin, Gluttony). It is mentioned that a person cannot physically eat themselves to death in one sitting, so Doe eventually deals a heavy blow to the man's stomach, causing it to burst.
- Arranging a fatal bloodletting of a rich attorney (representing the second sin, Greed).
- Tying up a drug dealer and child molester on a bed for a year, just about keeping him alive and eventually letting him die from shock (representing the third sin, Sloth).
- Forcing a man wearing a bladed S&M device to simultaneously rape and kill a prostitute (representing the fourth sin, Lust).
- Mutilating a young model's face to drive her into suicide (representing the fifth sin, Pride).
Mills and Somerset eventually track Doe to his apartment, unaware that Doe is out at the time. Doe then comes down the corridor and sees the two detectives at his front door, prompting him to fire upon them. Mills chases Doe out of the building but Doe catches him off guard and aims a gun at the side of his head. Mills is unable to identify the killer as his face is shrouded in darkness, but Doe leaves him alive and disappears.
Later, when Mills and Somerset return to the police station, Doe appears to them and intentionally gets himself arrested. The detectives are confused as to why Doe would hand himself in when he is two murders away from completing his "masterpiece", but Doe tells them that there are two more bodies hidden away. He then states that he will show them where the bodies are as long as Mills and Somerset are the only two to accompany him, otherwise he will plead insanity. The detectives agree and Doe directs them in their police car to the location of the bodies. Throughout the journey, Mills desperately and aggressively attempts to understand Doe's reasons for his actions, with Doe calmly stating that God told him to reveal to mankind what an awful and tragic place the world has become.
They arrive in a desert area in the middle of nowhere and a delivery van steadily approaches them, alarming Somerset. As he halts the van in its tracks, the driver claims that he was told to arrive at this location at this precise time to deliver a box. While Somerset looks at the box curiously, unsure whether he should open it or not, Doe tells Mills that he admires him. When Mills questions him on this, Doe reveals that he was jealous of his normal life (representing the sixth sin, Envy) and went to visit Mills' wife Tracy, attempting to "play husband" by raping her. Since this didn't work out, Doe decapitated her, and Somerset is horrified to find that the box contains Tracy's severed head. Somerset rushes back to Mills in an attempt to prevent him from killing Doe, knowing that is what the killer wants, but Doe provokes him further by telling Mills that Tracy was pregnant, as she had been keeping it a secret from him. Doe then awaits the inevitable punishment for his Deadly Sin, and unable to contain his fury and distress, Mills shoots him repeatedly (despite Somerset's objections). Even after his death, Doe's plan still succeeds since his masterful manipulation had prompted Mills to complete the seventh and final sin, Wrath. Following Doe's death, Mills becomes catatonic and is taken away by the police, much to Somerset's discomfort.
John was a lonely yet malevolent, twisted, sadistic, deadpan, cruel, God fearing vigilante who thought he was doing the right thing and flatly would not consider the possibility he was wrong. John was also calm, cool, collected and apathetic. He had a very gentle, smooth way of speaking and rarely raised his voice. He took it very poorly however when detective Mills suggested John's victims were "innocent", as John thought everyone who committed one of the 7 deadly sins was a criminal. John suffers from both borderline personality disorder and antisocial personality disorder.
John had begun life as an ordinary God fearing boy. Aside from having a violent, abusive alcoholic mother, John's life was pretty basic yet he took all the negativity his mother directed at him impassively. He had probably been bottling up his anger his whole life, which might've been what caused him to go over the edge when he could take no more. After having been cheated on by his wife, John started to become less unemotional about his rotten luck. After witnessing a car crash, John began to question if God was really watching over humans. The final straw was a relatively harmless occurrence, John found a book about the 7 deadly sins and something about them (the sins) caused him to finally snap and go insane. Since then, John dedicated himself to making a statement on the ugly side of humans by using the 7 sins against people who had committed them. John was one of those people who did not think he was crazy or evil, both of which he plainly was, because he felt he was working in God's best interest. John admittedly enjoyed the harm he caused his five victims, probably because he liked bad people (they were bad as far as he was concerned) getting what they deserved. Even though the subjects of Greed and Sloth were not ethically good people, John's method of dealing with them was demonic to say the least.
Despite being insane, John was far from unintelligent and was capable of working everyone involved in his scheme, victims and law enforcement, with no trouble. Were it not for John's tragic upbringing and marriage, he might have been capable of serving the community as a detective as he was incredibly cunning and fearless.