Assassination
There were three principal assassins involved in Sforza's death: Carlo Visconti, Gerolamo Olgiati and Giovanni Andrea Lampugnani, all fairly high-ranking officials at the Milanese court.
Lampugnani, descended from Milanese nobility, is recognized as the leader of the conspiracy. His motives were based primarily on a land dispute, in which Galeazzo had failed to intervene in a matter which saw the Lampugnani family lose considerable properties. Visconti and Olgiati also bore the duke enmity - Olgiati was a Republican idealist, whereas Visconti believed Sforza to have taken his sister's virginity.
After carefully studying Sforza's movements, the conspirators made their move on the day after Christmas, 1476, the official day of Santo Stefano, the namesake of the church where the deed was to be committed. Supported by about thirty friends, the three men waited in the church for the duke to arrive for mass. When Galeazzo Sforza arrived, Lampugnani knelt before him; after some words were exchanged, Lampugnani rose suddenly and stabbed Sforza in the groin and breast. Olgiati and Visconti soon joined in, as did a servant of Lampugnani's.
Sforza was dead within a matter of seconds. All the assassins quickly escaped in the ensuing mayhem save for Lampugnani, who became entangled in some of the church's cloth and was killed by a guard. His body soon fell into the hands of a mob, which dragged the corpse through the streets, slashing and beating at it; finally, they hung the body upside-down outside Lampugnani's house. The beheaded corpse was cut down the next day and, in an act of symbolism, the "sinning" right hand was removed, burnt and put on display.
Read more about this topic: Galeazzo Maria Sforza