Kingston Man Pleads Guilty to 2023 Fatal Shooting of Roommate in Mansfield

FALL RIVER (WBSM) — A Kingston man pleaded guilty to second degree murder and other charges in the 2023 fatal shooting of a 23-year-old man in Mansfield.

Bristol County District Attorney Thomas M. Quinn III announced that William O’Brien, 25, of Kingston pleaded guilty to second degree murder, and possession of a large capacity firearm and ammunition in Fall River Superior Court on September 10.

O’Brien was sentenced to life in prison with the possibility of parole at 22 years on the murder charge, and two and a half to five years concurrent on the firearms charge.

The Investigation and Discovery at the Mansfield Home

The charges date back to May 20, 2023, when at approximately 11:11 a.m., Mansfield Police were dispatched to 42 West Church Street after a reported shooting at the residence. Officers arrived at the single-family home and found Samuel Waters, 23, at the base of the inside staircase with multiple fatal gunshot wounds to his head.

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Cruiser Statements and Weapon Recovery Details

O’Brien was located a short distance away from the home, wearing a bulletproof vest. He was placed in the rear seat of a Mansfield Police cruiser equipped with audio and video recording, which captured him making what the D.A.’s Office called a “rambling statement” in which he admitted to killing Waters, described the weapon used, and where it was discarded.

Police searched a brook adjacent to the property and found a 9mm Polymer 80 semi-automatic pistol with a 31-round capacity. It was found loaded with 18 rounds of ammo.

The investigation revealed that Waters lived at 42 West Church Street along with O’Brien and several friends, all in their early 20s. Inside the home, police found multiple marijuana products, items that indicated a marijuana distribution operation, and multiple firearms.

According to the D.A.’s Office, O’Brien was seated on a couch on the first floor when Waters came down the stairs. At that point, O’Brien got up from the couch and started shooting Waters with “no clear triggering event established.”

Charges Came in the Days Following Mansfield Murder

On May 21, 2023, O'Brien was charged with the gun crimes in connection with the incident. The following day, he was charged with second degree murder.

“The defendant's statement in the police cruiser, which at times was incoherent, included that he believed the decedent was going to kill him,” the D.A.’s Office said. “There is no known evidence to support the defendant's stated belief. The defendant has a history of mental illness.”

D.A. Quinn's Statement on the Sentence

“This was a senseless killing of a young man who had his whole life ahead of him,” Quinn said. “The defendant clearly needs to be incarcerated for a very long time.”

Prior Criminal History and Legal Consequences

At the time of the murder, O’Brien was on probation out of Plymouth Superior Court for a firearms offense and had been sentenced to serve a two-year committed sentence. After the murder, while in custody, O’Brien assaulted a corrections officer and received a 90-day sentence for that crime.