In Black Mass, John Connolly is an FBI agent in the Boston of the 70’s. He’s sick of not being able to do anything against the Italian Mafia, so he figured out a way to end it, a desperate way We could say.
Whitey Bulger is an Irish Mafioso who has just gotten out of jail. John sees an opportunity in this, so he convinces the Irish mafioso to collaborate with him, as both the FBI and the Irish have a common enemy: the Italian Mafia.
Whitey Bulger accepts the deal and begins to collaborate. It is clear that the collaboration of the mafioso is not free, since it does in exchange for letting him do business in peace.
That unleashes a huge spiral of violence in the city that makes things worse than before the deal, and that makes Whitey Bulger become very powerful, so much so that nothing can be done against him.