The underworld says Bulger was being a rat for the FBI, but imo it would be more correct to say that it was the other way around: the FBI (Morris, Connolly, Rico etc) were "moles" for him in the law enforcement. In a documentary about his trial, it's said he didn't even really help with relevant information against rival gangsters, Flemmi provided more than him, so for the feds it's less embarrassing to say he was a rat, an informant for them and to reluctantly admit they gave him too much freedom, rather than acknowledging he wasn't that useful, because in that case they would have to admit they allowed him to stay free just in exchange of bribes or "old friendship" in Connolly's case, that would make them look even worse because, had Bulger really been a valuable informant, then his FBI handlers would be guilty of negligence and abuse etc, but if he wasn't, then they are corrupt "moles" at his service and accomplices to all murders he committed in even a more direct way.