Last Updated 12:15 p.m. ET
BOSTON A former hitman who admitted killing 20 people says he decided to testify against James "Whitey" Bulger after learning Bulger and his partner were FBI informants.
John Martorano served 12 years in prison after striking a cooperation deal with prosecutors. He was released in 2007.
Martorano, 72, took the witness stand Tuesday against Bulger. He described Bulger and his partner, Stephen "The Rifleman" Flemmi, as his "partners in crime," his best friends and the godfathers of his children.
Martorano says he decided to become a government witness against them because they violated his trust by becoming informants, something he said "sort of broke my heart."
Bulger, the former leader of the Winter Hill Gang, is charged in a 32-count racketeering indictment that accuses him of participating in 19 murders in the 1970s and `80s. He is also charged with extorting bookmakers, drug dealers and others running illegal businesses.
Martorano is one of three former Bulger loyalists who struck deals with prosecutors and agreed to testify against him. Flemmi, Bulger's former partner, and former Bulger lieutenant Kevin Weeks also are expected to be key prosecution witnesses.
In a 2008 interview with "60 Minutes," Martorano told Steve Kroft that he never kept count of how many people he killed. "Until in the end, I never realized it was that many," he tells Kroft. He said he confessed to 20 in court.
In the 1970s Martorano was partners with the Winter Hill Gang, led by Bulger and Stevie "The Rifleman" Flemmi. Their business: gambling, loan sharking, extortion and murder.
Martorano's specialty was conflict resolution.
"We had a lot of problems with people. And you know, you just killed them before they kill you. It's kill or get killed at times," Martorano explained.
"On one occasion, you walked into a crowded bar . . . and shot somebody. In broad daylight . . . with the policeman across the street," Kroft said.
"Correct," Martorano admitted.
Martorano told Kroft that John Connolly, a corrupt FBI agent who had been one of the top organized crime investigators in Boston, told them that an associate of theirs named John Callahan was about to rat them out on the 1981 murder of Roger Wheeler. In 1982 Callahan became Martorano's 20th victim.
"Do you think that John Connolly knew that you were gonna kill Callahan?" Kroft asks.
"Sure," Martorano says. "He said it, 'We're all going to go to jail the rest of our life if this guy doesn't get killed.'"
"And this an FBI agent telling you this?" Kroft asks.
"This is an FBI agent telling it to Whitey, telling it to me," Martorano says. [In 2008 Connolly was sentenced to 40 years in prison in connection with the Callahan killing.]
In 1995, Martorano learned that Bulger and Flemmi had been top-level FBI informants, snitching on the Italian mafia and on Martorano and other gang members. They had violated his code of loyalty, especially Bulger, he explained to Kroft: "I'll go along with a lot of things, but not -- no Judas, not no informant," Martorano said. "I never informed or ratted on nobody. And if I could've killed him, I would've killed him. But he wasn't there and that's what I think he deserves."
During his opening statement to the jury, Bulger's lawyer, J.W. Carney Jr., raised questions about the credibility of Martorano, Weeks and Flemmi, citing the "extraordinary" plea deals they struck with prosecutors.
"The federal government was so desperate to have John Martorano testify ... they basically put their hands up in the air and said take anything you want," Carney said.