Certainly the mobster who killed more people is Giovanni Brusca, here is his testimony:
''I killed Giovanni Falcone. But it was not the first time I had already used the car bomb to kill the judge Rocco Chinnici, and the men of his escort. They are responsible for the kidnapping and death of the young Giuseppe Di Matteo, who was thirteen years old when he was kidnapped and fifteen when he was murdered. I personally ordered and committed murders over 150 . Even today I can not remember them all, one by one, the names of those I killed. Many more than a hundred, certainly less than two hundred. "

http://en.wikipedia.org/wiki/Giovanni_Brusca