William J. O’Brien was sentenced Thursday in Fall River Superior Court to life in prison with the possibility of parole after 22 years in the 2023 killing of his Mansfield roommate, 23-year-old Samuel P. Waters.
Court records show that O’Brien, formerly of Manchester-by-the-Sea, pleaded guilty to second-degree murder, possession of a large-capacity firearm, and possession of ammunition without an FID card. He was credited with 390 days already served.
According to the Bristol County District Attorney’s Office, O’Brien, then 22, was indicted on August 17, 2023 and was arraigned that September in Fall River Superior Court and held without bail.
The shooting was reported at 11:11 a.m. on May 20, 2023, when a 911 call came in for 42 West Church Street. First responders found Waters already deceased.
NBC10 Boston previously reported that prosecutors said O’Brien was found in the street wearing body armor, fled when police arrived, and later told officers he apologized for killing Waters. Witnesses also identified him as the shooter.
An autopsy determined Waters died from multiple gunshot wounds to the head.
The Sun Chronicle reported that O’Brien was staying in the house Waters rented and that an eyewitness said he fired without speaking after an argument. They also reported O’Brien had a prior gun conviction, was on probation, and had a warrant at the time of the shooting.
A family member of Waters told Fall River Reporter the following after sentencing.
“My family and I are relieved that justice is served. Sam was a light in the life of every person that knew him. We miss him terribly.”
An obituary stated that Waters was known as a vibrant, happy person, with a playful spirit that everyone loved. All who knew him described him as generous, kind, easy to talk to, and fun-loving.
The case was prosecuted by Assistant District Attorney Patrick Driscoll of the Bristol County District Attorney’s Office.
