In an intense and sometimes bizarre verbal battle with defense attorneys for James “Whitey” Bulger, ex-hit man John V. Martorano insisted today he is “a nice guy’’ who should not be considered a serial killer, even though he has murdered 20 people.

“Were you a serial killer?” defense attorney Henry Brennan asked Martorano as the cross-examination of Bulger’s former henchman got underway in US District Court.

“No,’’ Martorano replied “Serial killers kill until they get caught or stop. I confessed my murders. Serial killers kill for fun. They like it. ... I never liked it. I never had any joy.”

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A skeptical Brennan kept up the sharp questioning.

“No satisfaction?” he asked.

“None,’’ replied Martorano.

Martorano, testifying for a second day, is a key prosecution witness in the long-awaited racketeering and murder trial of Bulger in US District Court in Boston. His testimony, elicited under a plea agreement under which he served only 12 years in prison for his murders, is crucial to the case because he was part of the inner circle of Bulger’s Winter Hill Gang.

But the story of his past crimes is violent and disturbing. And Martorano, whose victims included friends and innocent bystanders, is hardly a sympathetic figure — a fact that defense attorneys were apparently determined to highlight under cross-examination.

Martorano has already told jurors that he did not consider himself a hit man, and that he committed his murders to protect friends or family — not because he was paid to kill. Martorano testified that he had learned from nuns and priests to protect friends and family.

“I didn’t enjoy killing anybody,’’ he said. “I enjoyed helping a friend if I could.’’

He added, “I always tried to be a nice guy.’’

Brennan noted that during an interview with the “60 Minutes” television news show, Martorano had described himself as a vigilante

“Does that make you a vigilante — like Batman?” Brennan asked him sarcastically.

Martorano has testified that at least three of the people he murdered were innocent bystanders, including 19-year-old Elizabeth Dickson and 17-year-old Douglas Barrett, who were murdered on Jan. 6, 1968, because they were sitting in a car with Martorano’s target, Herbert Smith, 47.

print scan/ Bg Store **OPS** Dorchester Ma. 6/12/1975. fatal shooting of Eddie Connors in a telephone booth after he was gunned down gangland fashion on Morrissey Blvd. / bw
Martorano testified that Flemmi and Bulger shot Edward Connors in a phone booth on Morrissey Boulevard (Globe File Photo)
The Boston Globe

Today Martorano said he remained haunted by the deaths of Dickson and Barrett.

“I did feel bad. I still feel bad. It was the worst thing I did,’’ he told Brennan. “But I can’t change it.’’

He also told Brennan he felt bad about killing businessman John Callahan on Aug. 3, 1982, because he considered Callahan a close friend.

“When you said hi to your friend [Callahan] before you murdered him, did you look him in the eye?” Brennan asked.

“I did,’’ Martorano said. But “Mr. Bulger insisted on it, so I did it.’’

The verbal combat with Bulger’s defense attorney came after Martorano incriminated Bulger in a total of 11 murders, with Bulger committing one himself and playing a role in the others as a driver, bystander, or part of the cleanup crew.

Bulger is accused in a sweeping federal indictment of participating in 19 murders while at the same time under protection as a prized informant for the FBI, along with his former right-hand man, Stephen “The Rifleman” Flemmi.

Martorano testified today that Bulger and Flemmi shot and killed Edward Connors in Dorchester on June 12, 1975. It was the first time that jurors were told by a witness that Bulger himself had committed a murder.

Under questioning by Assistant US Attorney Fred Wyshak, Martorano described the decision made by six members of the Winter Hill Gang, led by Bulger, to kill Connors because they had been told that Connors had bragged how he had helped Bulger’s crew murder James O’Toole, a Bulger rival.

“We decided to shoot him,” Martorano said of Connors. “Whitey devised a plan.”

Martorano said he drove Bulger and Flemmi to Dorchester where they found Connors waiting for them.

“They walked to the phone booth and shot Eddie,’’ Martorano told jurors today.

Martorano said the two men got back into the car and told him briefly what happened.

“He’s gone,” Martorano said he was told.

Martorano, 72, who has already twice testified for prosecutors in other cases, also described how Bulger crafted a plan to kill Tommy King, whom Bulger disliked intensely.

“Him and Tommy couldn’t get along,” Martorano testified today. “He wanted to get rid of Tommy.’’

Martorano said Bulger knew that King would be suspicious of him or other members of the Winter Hill Gang. So they told King they needed his help to murder another man, which convinced King to get into a car and sit in the front passenger seat.

Flemmi arrived and handed out guns from a paper bag to King and the others. King’s gun was loaded only with blanks. Martorano said he sat behind him in the rear seat of the car.

“I shot Tommy,’’ Martorano testified. “Where did I shoot him? In the head.”

He said he got out of the car while Bulger and Flenmi drove off with King’s body still in the car.

Sometime later, Martorano said, he was riding in a car with Bulger as they drove over the Neponset River bridge connecting Dorchester and Quincy.

“Tip your hat to Tommy,’’ Bulger told Martorano.

King’s body was recovered from the marshes underneath the bridge in 2000.

Follow Globe coverage from the courthouse at boston.com’s live blog on the Bulger trial.