William S. O'Sullivan

In June 1972, Donald Killeen was murdered by Jimmy Mantville, another enforcer for the Mullen Gang. In the aftermath, the leadership of the Killeen crew devolved on Bulger. However, Bulger and the other Killeen loyalists fled Boston after the murder of their boss. Instead of murdering them as some Mullen gangsters wished, Mullen leader Patrick Nee arranged for the dispute to be mediated by Winter Hill Gang boss Howie Winter and Joseph Russo of the Patriarca crime family. After a sit-down in Boston's South End, the Mullen and Killeen gangs joined forces with Howie Winter as overall boss. In November 1974, Bulger, "took care of the right brother".

In February 1971, Buddy Roache, a close associate of McGonagle told Whitey Bulger and O'Sullivan that he was going to murder Donald Killeen, and if they interfered, he would kill them. A violent argument followed, Roache drew his weapon and O'Sullivan shot him. Roache was hospitalized but survived the near fatal gunshot wound. Following the shooting, both Bulger, O'Sullivan, and neighborhood boss Donald Killeen expected retribution for the attack on a ranking member of the rival Mullen Gang. This fear proved well-founded and the rivalry between the Mullens and Killeens exploded into a gangland war.

According to the memoirs of Mullen leader Patrick Nee, Paul McGonagle was thoroughly livid over the murder of his brother and, believing that O'Sullivan was responsible, received permission to kill him. On March 28, 1971, after having dinner with his wife at Jimmy's Harborside Restaurant, O'Sullivan dropped his wife off at their home and told her that he had an errand to do and that he would be back in an hour. Shortly after midnight, Mary noticed that his car was parked in the driveway in front of their house at the bottom of their driveway, it was surrounded by a group of neighbors. She went out to see what her neighbors were looking at and she saw her husband's bullet riddled body. She at first did not recognize his body but made positive identification from the suit that he wore. O'Sullivan had been shot by several gunmen numerous times as he sat behind the wheel of his car and was hanging half out of the car with the driver's door open.

O'Sullivan was born in South Boston. He moved to the Savin Hill section of Dorchester with his wife, Mary, with whom he had six children. He worked for the state Massachusetts Department of Public Works until he was injured on the job in 1970 working on an MBTA tunnel at South Bay, Boston in South Cove. He had also previously worked as a bartender in South Boston. His wife Mary later told reporters, "Bill was a kind father. He wasn't strict with the children. He like young people, however, he was against... long hair... he didn't think it looked right."

William S. O'Sullivan (January 31, 1928 – March 28, 1971) was an American loanshark and longtime enforcer for Donald Killeen, the head of the Irish mob in South Boston, Massachusetts. O'Sullivan also acted as a mentor to a young James J. Bulger, who grew up to become the leader of Boston's Winter Hill Gang.