In Black Mass Brian Halloran ran to the FBI after finding out about the killing of the owner of the sport team. He didn't have a problem killing people shown in his first scene and he knew about the plan when they were talking about it in the restaurant. He even got paid off to keep his mouth shut. So why did he run to the FBI as soon as he heard about the killing?
The film portrays Whitey Bulgar as over-protective suspicious control-freak. Bulgar constantly eliminated "loose-ends" who could testify or simply talk too much: people who did small job ends or knew crimes in what Bulgar have had hand in. Bulgar perceived such bendable people weak natured - and a possible threat.
His suspicions partly lies in 50s bank robberies, that he had been part of. He was caught, just because one of his accomplice in crime snitched him out.
Bulger said Smith "squealed on us" after he was caught. He states that he was caught in a nightclub in Revere, Massachusetts after being a fugitive for two months. He believes that a friend walked him into a trap.
He was sentenced to 25-years in prison, which he had to serve 9, just because someone squealed on him. Therefore his suspicion is understandable.
Halloran may have felt Bulgars extensive suspicion - so it would not be safe to do a job for him. But if you say no, it could result in similar trustworthiness from Bulgar. Halloran may have feared for is life, but him facing murder charge was main reason of his cooperation with FBI.
Halloran was facing a state murder charge and hoped his cooperation would help him in that case — and secure protection for himself and his family.