Bruce Greenwood was a detective with the NYPD.  Almost everyone who was in his unit was taking bribes from the owl which Jillette handed out to different cops in the unit.  Bruce's child had an accident and needed cash quickly so he started taken bribe money like other police officers in his unit.  Detective Greenwood also took the money so it wouldn't alienate him from his fellow officers.  Bruce did not considering taking bribes as wrong since it was normal in his unit since he and his fellow officers didn't have to do anything for the money besides stay away from the owl.  

Bruce Greenwood's partner Frank Castle however was disgusted by other officers taking money and was not quiet about it either. Jillette and others worried that the Punisher might turn them into internal affairs and that they might end up going to jail.  They started openly talking about murdering Frank Castle amongst themselves.  Bruce worried about his partner and best friend’s safety warned Castle that other officers were going to kill him if they thought he was going to talk to internal affairs.   

Frank upon hearing this information tells Bruce that he's already talked to internal affairs about the corrupt cops taking money.  Frank tells Bruce that "the people of this city deserve more.  Your family deserves more, Bruce."  Frank states that he will not have his son grow up and find out that his father took money and end up hating for the rest of his lie.  He tells Bruce that he turning himself in is the right thing to do that his daughter will forgive him and be proud that he did the right thing in the end.  

Right after talking to Frank Bruce tells Jillette everything that Frank told him.  Jillette than goes and tries to kill Frank Castle but only wounds him and ends up killing his family instead.  Frank then goes after the dirty cops who killed his family and other criminals calling himself the Punisher.  When Bruce finds out that Frank is killing those involved with his family's death he goes to see his lawyer Matt Murdock.  He confesses everything to him needing to get the guilt off his shoulders.  After he is finished the Punisher snipes him from a nearby building killing him ending his guilt.