If I remember well, in the documentary "United States vs James Bulger" the defense tried to prove he wasn't an informant, while the FBI insisted he was. Because if he was an informant, the FBI could try to "justify" their giving him considerable freedom of action by saying "oh well, at least he was useful because he gave us information to put away criminals more dangerous than him". On the other hand, if he wasn't an informant, it makes the FBI look even more like shit, because that would mean they covered up and took bribes for a criminal who even didn't help them to make arrests and solve cases, meaning they would look like dirty/crooked ones, plain and simple, who did help Bulger purely for financial gain or personal favors.

Willie Marfeo to Henry Tameleo:

1) "You people want a loaf of bread and you throw the crumbs back. Well, fuck you. I ain't closing down."

2) "Get out of here, old man. Go tell Raymond to go shit in his hat. We're not giving you anything."