Convicted mob hitman Fotios "Freddy" Geas, the reported suspect behind the killing of gangland boss James "Whitey" Bulger in a West Virginia prison, probably had a simple motivation, said Ted McDonough, a private investigator who knows Geas well.

“Freddy hated rats,” McDonough told the Boston Globe. “Freddy hated guys who abused women. Whitey was a rat who killed women. It’s probably that simple."

Bulger rose to power as a secret informant to the FBI and agents looked the other way while he committed murders, extortion and other crimes.

Geas, 51, is serving a life sentence in high-security USP Hazelton prison in Bruceton Mills for his role in the killing of Adolfo "Big Al" Bruno, the one-time head of the mob in Springfield, Mass.

The man who ordered Geas to kill Bruno and a hitman hired by Geas to commit the murder testified against him. Geas harbored a special hatred for informants -- criminals who save their own skin by giving up others to authorities.

He once pleaded guilty to threatening to kill a witness against his brother, Ty.

Sources told the newspaper Bulger had been murdered by more than one inmate and that Geas didn't dispute his role in the killing.

David Hoose, the lawyer who represented Geas in the mafia killing, said he wasn't surprised to learn his former client refused to identify possible accomplices.

“He wouldn’t rat on anybody,” Hoose said, “and he had no respect for anyone who would.”

In this 1995 file photo provided by the FBI, fugitive mobster James "Whitey" Bulger is shown in a photo released Saturday, April 17, 2004, and taken shortly before he disappeared in 1995.

In this 1995 file photo provided by the FBI, fugitive mobster James "Whitey" Bulger is shown in a photo released Saturday, April 17, 2004, and taken shortly before he disappeared in 1995.

Bulger, 89, was recently moved to the prison from another correctional institution in Florida. He was once one of America's most wanted criminals. In 2013, he was convicted of a slew of crimes after 16 years on the run. The jury believed he participated in 11 of 19 killings.

No one has been charged in his killing.

Geas has a long reputation for a propensity for violence. In 2006, he went to jail for beating a pair of men with a baseball bat at a strip club.

McDonough described him as extremely personable, but a committed criminal.

“A good conversationalist,” McDonough said. “He liked me because I was his private investigator. You would not want Freddy as an enemy.”

He suspected Geas's stature would be elevated by killing Bulger, especially among mobsters, who would have felt a sense of duty to take out Bulger themselves after learning he talked to the FBI about the Boston mob.

Local authorities knew the Geas brothers as hired muscle for Mafia leader Anthony Arillotta. Arillotta eventually testified against Geas for the 2003 killing of Gary Westerman and the Bruno hit.

"Freddy is a man's man," Springfield attorney Daniel D. Kelly told MassLive.com. "After Anthony Arillotta flipped, there was a back channel for Freddy to try to persuade him to cooperate too. He didn't even blink an eye. He didn't flinch. He just said no."

Kelly represented Geas in multiple criminal cases and is friends with him to this day.

.An original photo released by the FBI in 1984 shows James "Whitey" Bulger

.An original photo released by the FBI in 1984 shows James "Whitey" Bulger ((AP/FBI))

Tom Donahue, whose father, Michael, a truck driver murdered by Bulger, was comforted to know Bulger experienced the pain so many of his victims had.

“I think it’s justice,” he said, adding that he hoped to put money in Geas's prison canteen account.