Boston - A former enforcer for James "Whitey" Bulger who admitted killing 20 people insisted Tuesday that he is not a hit man or a serial killer, but instead is a "nice guy" who was only trying to help his family and friends when he pumped bullets into victims while working with Bulger and his gang.

John Martorano made the statements in his second day on the witness stand during an aggressive cross-examination by a lawyer for Bulger, who is charged in a racketeering indictment with participating in 19 killings in the 1970s and '80s as leader of the Winter Hill Gang.

Bulger's lawyer, Hank Brennan, went after Martorano, sarcastically asking him about an assertion that he did not consider himself a hit man.

Brennan asked Martorano whether mass murderer or serial killer were more appropriate descriptions for him.

"You're different from a serial killer how?" Brennan asked.

"A serial murderer kills for fun. They like it," Martorano said. "I don't like it. I never did like it."

Martorano served 12 years in prison after he cut a deal with prosecutors and agreed to testify against Bulger. He is one of three former Bulger loyalists who are expected to be the prosecution's star witnesses against Bulger.

Bulger fled Boston in 1994 and was one of the FBI's most wanted fugitives for more than 16 years until he was captured in Santa Monica, Calif., in 2011.

In testimony Monday, Martorano said he decided to become a government witness after learning that Bulger and Stephen "The Rifleman" Flemmi had been working as FBI informants.

Bulgers' lawyers deny that he ever provided information to the FBI. In opening statements to the jury last week, attorney J.W. Carney Jr. said Bulger paid FBI agents to tip him and his gang about investigations so they could avoid prosecution.

Martorano said he killed people when they hurt or threatened his family, or if they threatened to tell authorities about the gang's illegal activities. He said he always tried to help people he was close to, either by giving them money or in other ways.

"I always tried to be a nice guy," he said.

But Bulger's lawyer grilled Martorano about several instances where he killed the wrong person or innocent people who were with the intended target.

Brennan asked Martorano whether he regretted killing a 19-year-old girl and a 17-year-old boy who were in a car with Herbert Smith, a man who had beaten up Flemmi, when he jumped into the car and shot all three.

Martorano said he saw three silhouettes as he approached the car. Since he expected Smith to be alone, he thought Smith may have planned to ambush him when he got in the car, so he killed all three people, he said.

"I did feel bad. I still feel bad. It's the worst thing I did, but I can't change it," he said of the 1968 killings.

Bulger glanced briefly at Martorano as he took the witness stand for a second day. Before he testified Monday, the two men had not seen each other since 1982, Martorano said.

Brennan questioned Martorano extensively about the killing of John Callahan, a Boston businessman whom Martorano described as a close friend. Martorano said he reluctantly agreed to kill Callahan at the insistence of Flemmi and Bulger, who said Callahan would likely finger the gang in the 1981 killing of Tulsa, Okla., businessman Roger Wheeler.

Martorano testified earlier that he waited in the parking lot of a Tulsa country club until he saw Wheeler get in his car, then shot him between the eyes.

"I agreed to go along with (killing Callahan) because they were my partners and I couldn't vouch for him not getting everybody in trouble," Martorano said. He said he offered to pick Callahan up at a Florida airport, where he killed him.

"Did you look him in the eye?" Brennan asked Martorano.

"I did," Martorano replied.

He said he told Callahan to sit in the front seat, then he got in the back seat and shot Callahan once in the back of the head.

The defense is set to continue cross-examining Martorano on Wednesday.