Was Lefty going to kill Donnie before the FBI showed up at the end?
When Lefty took Donnie to do the hit on Bruno Red near the end of the movie, he says that Donnie has to do it and has a gun in his hand as his is pushing Donnie to shoot Bruno, this is when the power to Bruno's boat has been turned off, then suddenly the FBI showed up out of nowhere and escorted Donnie away and arrested Lefty.
Later on Lefty might have realized that Donnie was an agent because he was not handcuffed and walked off with the agents, but it appeared that Lefty might have been about to whack Donnie unless Donnie went through with his request to kill Bruno.
How bad would it have been if the FBI had showed up a few minutes later and Lefty got pissed off at Donnie and whacked him after it was obvious that Donnie wouldnt kill Bruno before the FBI could get there to intervene?
And what would happen to Donnie if he had agreed to kill Bruno because he liked Lefty and Bruno was trying to kill Lefty and his faction during the mafia war?