Dr Miles Warren's obsession with cloning likely was a long time coming. Nothing is known about his childhood but it is known that he was a brilliant biologist who was also a professor at Empire State University. He was married and had two children. His family was his life, until he became more interested in his work. This caused friction in his marriage and Monica left with the children. Soon after, they were killed in a car accident. He mourned deeply for his family, until an attractive blond student named Gwen Stacy enrolled into one of his classes.
He came to love Gwen and was devastated when she was killed by the Green Goblin. Soon after her death, his assistant, Anthony Serba, made a break through in cloning. He gave some of Gwen's cells (which had been saved for a class experiment) to Anthony, telling him that they were frog cells. When Anthony discovered the cloned cells were human, he tried to destroy them and Miles killed him in a fit of rage.
Miles' psyche broke down and he blamed the murder on an alter ego, the Jackal. He claimed he worked feverishly over the next few months: caring for the clones, training himself athletically and making a Jackal costume and equipment. He planned a revenge in which a Spider-Man clone would kill the real Spider-Man, the man whom he blamed for Gwen's death. The plan backfired and Warren was seemingly killed. (Amazing Spider-Man #149)
Years later, the Jackal returned with a genetically enhanced body instead of a costume. He also returned with several helper-clones. (The man that had died years ago was a clone.) By this time, he knew that Peter Parker was Spider-Man and still blamed him for Gwen's death. He also had a plan to wipe out humanity with a virus and replace them with clones. Clones which were genetically perfected and obedient to him.
The Jackal toyed with the Spider-Men , telling them that he was their father and they were his sons. (The Spider-Man clone from years prior was alive and well, wandering the Western US.) He also kept them guessing as to which of them was real. He tried to make his move for world domination but fell to his death while trying to save the Gwen Stacy clone. (Maximum Clonage Omega)
A Sudden Snap, or a Slow Burn?
To hear Professor Warren describe what happened, the events that changed him from university professor to megalomaniac all seemed to happen very quickly. Still, Warren has been proven to be a liar, to others and to himself. He lied to Spider-Man and his clone as to which of them was real and he lied to himself, saying that he never had any children, which is why he felt so protective of Gwen when she entered his class. "Perhaps she reminded me of the daughter I never had," he said. (Amazing Spider-Man #149) Yet, he did have a daughter, one he loved and mourned deeply for after she died. (Web of Spider-Man #125)
After Anthony was accidentally killed, Warren went about making a costume and equipment. He says it was a matter of months from Gwen's death to his attempt to kill Spider-Man with his clone. In those same months, he supposedly also built the regeneration tank in which he changed his body to that of a super human. By his own admission, he was obsessive about his work before his wife and children left him. He likely had some kind of plans long before his family fell apart, before Gwen died and before he became the Jackal.
A Mind at War with Itself
What is the reason that Miles became the Jackal? To avenge Gwen Stacy and bring her back to life. He loved Gwen, even if he couldn't admit it right away. On one hand he called her his daughter and on the other he stammers on his exact opinion of her. "The re-creation of a woman I--that is, a girl I felt, I--the re-creation of Gwen Stacy--and the acting out of my revenge for her savage slaying!" (Amazing Spider-Man #149) What was his second goal? Wipe out humanity and replace them with obedient clones. People that he could control. Warren couldn't control the people in his life. His family distracted him from his work and then left him. A costumed maniac killed his new love. The world seemed like a chaotic place. Hence, he would start over with humanity and make them obey his whims.
Despite this need for logic, he had romantic feelings for a student of his, which the board of directors at E.S.U. would doubtless have seen as unethical, had he acted on those feelings. He also killed his lab assistant in a fit of rage. Not exactly a rational thing to do. He couldn't control other people or himself. He blamed his murder of Anthony on the Jackal, in an attempt to make sense of what happened. He also likely fed his lust for Gwen into the Jackal personae. What Dr. Warren couldn't get away with, the Jackal could. Warren likely heaped all of his inappropriate emotions onto his alter ego.
Psychopathology: A Smorgasbord
The Jackal has always been an obsessive worker and quick with a quip. He worked on his new criminal enterprise for months at a time. This kind of non stop energy and heightened mood are symptoms of mania. When a person has a manic episode, they feel invincible and engage in various pleasure seeking activities. Mania is caused by a chemical imbalance in the brain, often resulting from genetics and environmental factors. It also makes a person emotionally unstable. Upon discovering the existence of a new Green Goblin, Jackal goes berserk and starts smashing up his lab. He wonders if someone is making clones behind his back. "I should be devoting all my time--with a few quick breaks to catch the new Star Trek series--to wiping out mankind, but nooooo!...It's no use! I'll never be able to concentrate on my real work until I have an answer!" (Spectacular Spider-Man #125)
His devotion to "wiping out mankind" is also a symptom of narcissism. He creates a perfected body for himself (granted, its covered with green fur) and considers himself to be the leader of a future world. All in all, he considers himself to be overly self important. Most super-villains hire a gang of thugs to do their dirty work, but Warren doesn't even bother. In his latest scheme, all of his assistants are clones of himself. If you want something done right, you do it yourself. (Amazing Spider-Man #673)
Next, he also shows signs of obsessive compulsive disorder. He must have focused entirely on becoming the Jackal in the months after Gwen's death, neglecting his duties as a university professor in the process. Its also hard to see how he had time to eat and sleep with all of that work to do. A healthy person would have mourned the loss of his family and student and moved on, not devoted their life to cloning his illicit love.
Finally, he suffers from a split personality. Miles was under a great deal of stress when he first started thinking about the Jackal. His family was dead, his new love was dead and he had just killed his laboratory assistant. Some researchers say that a split personality (known in psychiatric circles as dissociative identity disorder) stems from childhood abuse, in which the sufferer walls off the painful memories. These memories can form another personality. Miles may have been blocking out unpleasant and inappropriate thoughts and memories for years, but it is certain that he had walled off the death of Anthony Serba from his memories. As the Spider-Men fought soon after the cloning began, Miles' clone comes to his senses and realized "Dear Heaven! I killed Anthony Serba--not the Jackal! I--I couldn't face that--hid it from myself!" (Amazing Spider-Man #149)
Miles Warren's habit of lying to himself and closing off unpleasant memories led to the development of the murderous second personality, the Jackal. Perhaps he had been abused as a child and thus began the trait of forced forgetting, but regardless, all of that emotion seethed inside him and burst out after a trio of tragedies. He likely had some psychological issues before he ever put on the green costume, as he obsessed over his work, at the expense of his marriage and family. This obsessive compulsive aspect of his personality was carried over into his plans for world domination and for his student, Gwen Stacy. Also carried over from his previous life is the need for order and control, which is still a major part of his motivation, despite the manic switches of mood. He saw the world as having gone mad and in his desire to control the chaos, he went mad himself.