Eddie Brock was a journalist and photographer, his biggest scoop--the identification of a serial killer--was destroyed when Spider-Man caught the real killer.
His resentment over missing his big moment made him a welcome and willing target for the alien symbiote that was Spider-Man's black suit. Brock walks a fine line between hero and villain, often fighting against other symbiotes (and their gods).
Carl "Crusher" Creel was a boxer. He also had a history of criminal activity, which led him to prison. When he drank a potion given to him by Loki, he developed an amazing superpower.
Creel discovered that he could absorb the quality of any material he touched and reproduce it in his body. Thus the Absorber was born. Creel is primarily a villain in the comics, though at times he has been something of an anti-hero. However, he tends to stay on the criminal side.
A genius professor who cannot be described as an ordinary man. His genius played a very bad joke on him. One day he designed and created mechanical tentacle arms, which he decided to test on himself. Naturally, things didn't go according to plan...
Due to an electrical short circuit, the artificial intelligence of the mechanical arms penetrated Professor Octavius' body. After that, Otto started running criminal enterprises. The psychological trauma of his childhood further aggravated his situation.
Baron Zemo comes from a rather famous family in his native Germany, but was otherwise an ordinary man before he became a supervillain. Helmut Zemo worked as an engineer, going about his life, although he picked up quite a few racist lies from his father, who held the title Baron Zemo during World War II.
When his father died, Helmut decided to go after Captain America, whom he held accountable. Zemo put his scientific and engineering know-how to work, committing a series of terrible acts that his father would never have dreamed of.
Sandman is one of Spider-Man's oldest villains. Flint Marco started out as a famous high school soccer player who got involved with some gangsters. From that point on, his life took a sharp turn and turned into a crime spree.
He escapes from prison to a nuclear test site where somehow he gets super powers. His body can turn into sand and back again, Marco returns to New York, determined to use his new ability to strengthen his criminal gang. He later becomes one of the founding members of the Sinister Six.
The Taskmaster is secretly Anthony Masters, a former S.I.T. agent In addition to his skills as an agent for an elite organization, Masters was an ordinary man until he ingested an experimental serum developed by Hydra.
The serum not only enhanced his physical abilities, but also his mental acuity; he was able to acquire knowledge instantly. He could also reproduce anything he saw, which allowed him to defeat Marvel superheroes. Unfortunately, a side effect was amnesia. Masters became a Taskmaster while still working for S.I.T.
Max Dillion worked as a lineman for the Electric Company in New York City. His normal life was shattered when lightning struck him at work, giving him the ability to control and generate electricity.
Electro immediately went over to the side of evil (because that's where the cookies and money are), his powers allowing him to do the outrages with near impunity. Electro is likely to be a member of the eventual Evil Six.
Cain Marko was a fairly ordinary kid when Charles Xavier came into his life. Marco's father became Xavier's stepfather and seemed to value Charles more than Cain. This pain and resentment accumulated in Cain for years, until one day he stumbled upon the Scarlet Stone of Cittorac. Once he got hold of it, he turned into a Juggernaut.
Juggernaut is one of the biggest and most powerful villains in the entire universe. He is also one of the most popular.
Mac Gargan was a private investigator hired by John Jameson to find dirt on Spider-Man.
What seemed like a simple enough job quickly spiraled out of control when Jameson talked Gargan into becoming a willing subject for an experiment that turned him into Scorpius. Gargan plunged into a life of crime, fighting Spider-Man and other Marvel heroes.
Bolivar Trask is not a supervillain, but he is responsible for some of the most destructive villains of all time: the Guardians. Trask created the "guards" to combat what he perceived as an existential threat of mutants.
Before that, Trask was an anthropologist. His fear of mutants increased with the realization that both of his children were mutants themselves. He concealed this fact and then began to promote a campaign of hatred and fear against mutants, though he eventually recanted. However, the damage was done.