Boston (CNN) -- With words as ammunition, a defense lawyer fired a machine gun burst of questions at the government's star witness Tuesday in the federal murder trial of reputed Boston mob boss James "Whitey" Bulger.
"You killed friends?" Bulger attorney Hank Brennan shot.
"Yes," replied prosecution witness John Martorano.
"Yes," said Martorano, who has admitted to 20 murders and who earlier served 12 years in prison on a government deal for his cooperation in the case against Bulger.
Martorano acknowledged he didn't like the term "hitman" since he "wouldn't accept money" for the killings he committed. And he countered the prosecution's descriptions of him as a "mass murderer" and a "serial killer."
"A serial killer kills because they like it. I didn't like it. I never had any joy in it," said Martorano, now 72.
Prosecutors say Martorano killed at the direction of Boston's Winter Hill Gang and Bulger, who is standing trial on 19 murder charges along with racketeering and extortion charges. Bulger was caught in 2011 after spending 16 years as a fugitive and landing on the FBI's most wanted list.
"My father, the priests, the nuns" taught him to always "defend family and friends," Martorano testified. When Brenann asked whether there was "any honor or integrity in what you did," Martorano replied, "I thought so."
Under questioning by prosecutors, Martorano explained that the six partners who comprised the Winter Hill Gang -- including Bulger, Steven Flemmi, and Martorano -- had to agree on business decisions and targeted hits. Flemmi is serving life terms without parole but avoided a possible death sentence by cooperating with federal authorities.
"Was it a democracy?" prosecutor Fred Wyshak asked, referring to the Winter Hill Gang. "So to speak," answered Martorano.
The former mobster described Bulger's role in six more murders Tuesday, implicating him in a total of 13 killings in just two days of testimony.
In one case, Martorano claimed that Bulger fired the fatal shot that killed one of the gang's targets, a Dorchester, Massachusetts, bartender who was "squealing" about a murder he had helped the crew commit.
That testimony marked the first time that Bulger was placed on the firing end of a gun in a killing.
In another slaying, Bulger sat at the wheel of a car, while another crime associate, Tommy King, sat in the passenger seat, with Martorano behind him, the witness said.
All had guns but "Tommy's had blanks." When the car pulled out "I shot him in the back of the head," Martorano testified.
"I was driving over Neponset bridge (in Boston) one day and Whitey said tip your hat to Tommy .... He's over there," Martorano testified, acknowledging that he knew where King's body had been dumped.
Martorano said that Bulger's "plan B" to take the heat off the Winter Hill Gang for King's murder was to kill King's good friend, Buddy Leonard, and put him in King's car to make it look like the two got into a dispute that ended with Leonard's death and King's disappearance. Leonard subsequently was found shot to death.
Another time, Bulger sat at a table with another crime associate the partners had decided to kill. As planned, Bulger was counting money with the man when Martorano walked in and shot the man in the temple.
In 1982, Martorano testified, he felt "really bad" about it but carried out a killing anyway. Outnumbered in the decision by his partners, he killed his friend John Callahan.
That came about after another killing: Callahan wanted to buy a business from Oklahoma businessman Roger Wheeler. When Wheeler wouldn't sell, Callahan asked Martorano to kill him.
Martorano said he flew to Oklahoma, picked up a suitcase with machine guns, revolvers and masks that had been shipped from Boston via Greyhound bus, and shot Wheeler between the eyes. Callahan was so grateful he paid his friend $50,000.
But when trusted FBI contact John Connolly indicated that Callahan was going to be pressured by the government to testify about the murder, the Winter Hill partners, with Martorano outvoted, decided it was too risky for Callahan to live.
"I felt lousy but these were my partners. We were up to our necks in murders already," Martorano testified.
He said he arranged to meet Callahan in Florida, picking him up at the airport and helping him with his bags. Then as Callahan got into the front seat, Martorano got in behind and shot his friend in the head.
"I felt really bad. I'd just killed a man I'd risked my life for in Oklahoma," he testified.
Asked by the prosecutor whether he regretted his life of crime, Martorano responded, "Who wouldn't?"
During Martorano's testimony, Bulger sat ramrod straight and kept his eyes focused ahead, barely moving. Bulger turned his head, seeming to look at his former friend sitting just six feet away, only when Martorano said he learned Bulger was an FBI informant.