It is said that the man now known as Dr. Mundo was born without any sort of conscience. Instead, he had an unquenchable desire to inflict pain through experimentation. By the time he was five, most of the pets in the Zaun neighborhood where Mundo grew up had gone missing. By his teenage years, his parents were nowhere to be found. By the time he had legally acquired his license to practice medicine, he had been acquitted of thirty-eight separate charges of murder by the Zaun authorities; the lack of evidence made prosecution impossible.
Dr. Mundo has become equal parts serial killer and mad scientist, though no one is entirely sure how his butchery qualifies as science. However, he has made tremendous strides in mapping the pain response in the human brain and body, going so far as being able to suppress it, even in the most excruciating of circumstances. He has also tapped into the primal parts of the brain through chemistry, learning how to enhance aggression and adrenaline, as well as dulling conscience and the survival instinct. In short, Dr. Mundo's life's work has been how to create the perfect science-enhanced killer. Unfortunately, the city-state of Noxus regards such behavior as a sign of initiative and ambition, rather than inhumanity. Originally fighting for Zaun, Dr. Mundo was recruited to also fight for Noxus in the League of Legends; the Madman dual faction status represents the fruits of a blossoming relationship between Zaun and Noxus. He continues his experiments to this day, even using himself as an experimental subject, as evidenced by his disfigured appearance and his... unique manner of speaking. There are rumors that the High Command in Noxus has given him free reign to pursue his life's work in his spare time.
Source: The Game Website.