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.