The top suspect in the killing of Boston gangster James “Whitey” Bulger is a former Mafia hitman serving a life sentence, according to a new report.

Fotios “Freddy” Geas — a career criminal and prison lifer at Hazelton federal penitentiary — is being eyed as one of the inmates potentially responsible for bludgeoning Bulger to death, and he hasn’t disputed his role in the murder, sources told the Boston Globe.

“Freddy hated rats,” private investigator and Geas’ friend Ted McDonough told the paper. “Freddy hated guys who abused women. Whitey was a rat who killed women. It’s probably that simple.”

Geas was most certainly aware that Bulger, 89, secretly served for years as an FBI informant while also running Boston’s vicious Winter Hill Gang from the 1970s through the ’90s.

Bulger, who had been transferred to Hazelton just hours earlier, was beaten to death Tuesday by more than one inmate, with one of his attackers using a sock with a padlock in it as a weapon, sources told The Post.

Geas, 51, is serving a life sentence at the maximum-security prison for his role in the 2003 assassinations of Adolfo “Big Al” Bruno, onetime head of the mob in Springfield, and associate Gary Westerman.

The lawyer who represented Geas in the Mafia killings wasn’t surprised to hear his former client didn’t dispute his role in Bulger’s murder and that he refused to identify any accomplices, the Globe said.

“He wouldn’t rat on anybody,” said attorney David Hoose. “And he had no respect for anyone who did.”