Norman Osborn IS the Green Goblin
Norman Osborn was the son of the wealthy business man Amberson Osborn. He had a decent childhood until Amberson lost most of the family fortune after the failure of his invention company, which had Amberson becoming alcoholic and violent. Young Norman and his mother would often be the targets of physical and psychological abuse by Amberson to compensate for his failures. This abuse had Norman realize that only the strong survive and that he will never be weak. This realization caused him to kill the family's dog, seeing it as one less problem in his life.
On one stormy and thunderous night, in order to cure his son's fear of the dark, Amberson locked Norman alone in one of the family's dark empty houses where Norman encountered and feared a "green, goblin-like" monster stalking him. Alone in the dark, Norman feared the monster would eat him once the darkness is replaced by the light, so he came to grip with the darkness, wishing it would never leave him. That night, he realized that the Darkness is better than the light.
Encounters like these made Norman develop a need for power and wealth so he would not become a failure like his father. He sought to regain the wealth he once had and he would stop at nothing to reach his goal.
Norman attended Empire State University with a science scholarship and received degrees in chemistry and electrical engineering. While at ESU, he met and fell in love with a young woman named Emily. Despite his hunger for power and wealth, Emily was able to provide some balance in the young man and bring out the best in him. The two would marry and have a son, Harold Osborn. Norman loved Emily tremendously, but the good times would not last and Emily died shortly after Harry's birth. Norman is left a miserable, misguided, insane, single father. Without Emily there to give his life any light, he re-dedicates himself to wealth and power, neglecting Harry in the process.
After graduating in business and chemistry majors, Norman founded his own company, OsCorp, with his old science professor, Dr. Mendel Stromm. He would become successful and wealthy yet a ruthless businessman and always power hungry. When Stromm borrows money from the company accounts, Osborn seizes the opportunity and turns Stromm over to the police for embezzlement, leaving Norman, the sole owner of the company.
With Stromm now in jail, Osborn decides to continue working on the strength-enhancing formula Stromm had been working on in order to sell it to the highest bidder. Osborn tries to re-make the formula following Professor Stromm's notes. In order to perfect the formula, Osborn uses his employees as test subjects. One of those employees is Nels Van Adder, who went on a killing spree as the red, goblin-like monster called the "Proto-Goblin" who attempted to kill Osborn at his laboratory but was saved by Arthur Stacy and George Stacy. After the failed attack, Norman finds more of Stromm's notes in his old desk.
He is certain the new notes are the missing piece to the formula, however when Osborn uses the new information, the mixture turns bright green and explodes in his face. He is hospitalized, but recovers at an incredible, super-human rate. He realizes the explosion not only increased his strength, but also his intelligence. This pushes the megalomaniac Norman Osborn over the edge, and his sanity is lost.
Stan Lee had originally intended to have the Green Goblin an ancient mythological monster found in an Egypt-like sarcophagus. However, Steve Ditko re-imagined the Green Goblin and turned him into a human instead. The Green Goblin's whole conception was all Ditko's idea. The Green Goblin debuted in Amazing Spider-Man #14 and his secret identity wasn't known but he became instantly popular. Lee and Ditko disagreed on the character's alter-ego; Lee wanted it to be someone Spider-Man knew but Ditko preferred it be a new character to introduce a sense of realism to the stories. According to Ditko, he had intended to have Frederick Foswell be the Green Goblin. This disagreement may have led to Ditko leaving Spider-Man and in issue #31, the Green Goblin was finally revealed as Norman Osborn.