Originally posted by gretzy
how did the professor become a villain?

Dr Connors, like his friend Dr Octavius, became the Lizard during an accident on his research of re-growing limbs using DNA or cell samples of lizards. If you can notice on the movie, spiderman 2, he has only one arm. Because of this paralysis, he was bent on proving that humans can re-grow, lost or damaged limbs through genetic means and this became an obsession to him more likely to follow the path/destiny of his friend Doc Ock.