Weeks said he helped Bulger even after Bulger fled Boston in 1994 when he learned he was about to be indicted.
That changed, Weeks said, when he learned that Bulger and Flemmi had been FBI informants for years. He said that went against the South Boston culture to never rat on your friends or your enemies.
Carney asked Weeks if he was concerned that he'd be seen as a rat after he made his deal with prosecutors to testify against Bulger and Flemmi.
"You can't rat on a rat," Weeks said.
Weeks described three killings he said he saw Bulger commit and several others he said Bulger admitted he had somehow engineered.
He said he was at a South Boston house with Bulger in 1985 when he heard that 26-year-old Deborah Hussey, the daughter of Flemmi's longtime girlfriend, would be coming over.
While Weeks was upstairs, he heard a thud coming from below him.
"Jim Bulger had her on the ground, choking her," he said.
Flemmi carried her to the basement but thought she was still alive. Flemmi wrapped a rope around her neck, put a stick in it and strangled her, Weeks said.
He said Flemmi then removed her teeth and they buried her under the dirt floor.
Hussey was one of three people who were buried in the same house after Bulger killed them, Weeks said. He said he helped moved the bodies later when the house was being sold. In 2000, after he began cooperating with the government, he led authorities to the new location in the Dorchester section of Boston.
Weeks said John McIntyre was killed after Bulger heard he may have spoken with authorities about a failed bid to send guns to the Irish Republican Army. He said McIntyre was chained to a chair and interrogated by Bulger, who held a machine gun.
After McIntyre admitted he was cooperating with law enforcement, Bugler wrapped a rope around his neck and tried to strangle McIntyre, Weeks said. The rope was too thick and the pressure caused McIntyre to vomit, he said.
"Jim says to him, 'Do you want one in the head?' and he said, 'Yes, please,'" Weeks said.
Bulger then shot him in the head, Weeks said. When that didn't kill him, Bulger shot him several more times, Weeks said.
During cross-examination, Bulger's lawyer focused largely not on the crimes Bulger is charged with, but instead on trying to get Weeks to acknowledge that Bulger loathed anyone who was a rat.
Carney asked Weeks if, during the two decades he spent working with Bulger, he made it clear that "what he hated above all else was informants."
"We killed people for being informants," Weeks said.