Whats the climax of the movie "The Departed"??..?

Question

Well...Frank Costello (Jack Nicholson). ..is showed as an Irish Mobster and Sullivan(Matt Demon) is a goon planted by him in the FBI...Now when Sullivan(Matt Demon) shots him(Costello) he mentions that Costello being a FBI informant...
So what was all this??...And what is the job of the Yellow envelope which the lady had..??
How does Sullivan gets to know that Costello was a FBI informant..???
And even when Costellos man is dyieng on the bed and he calls Billy (Leonardo) to ask him "How did u land up on the right address even when I gave u a wrong 1"...So he must have got to know who the rat was... So why didnt he killed him(Billy) or informed Costello?..
So the question still remains...If Costello was working for the FBI then for whom was Matt Demon working?...
Either I missed some part of the Movie...since I watched it on the CD ..or I havnt got it..
By all means can anyone help me?

Answer 1

First of all, that was no movie for CD/DVD, you should have been in the theather. Secondly, Matt Damon shot Jack Nicholson because he knew the man was surrounded by the FBI and they were going to take him in, he als knew the old still had trumps to keep himself out of jail and what they were(he was one of them), so to save his own bacon he killed the old man. The yellow envelope that Leonardo gave the girl was the proof that her boyfriend(Damon) was on the take. He knew she was the one person he could trust. He knew it from the information that everyone had to give Jack Nicholson that day. The full name and bank???? and he found it when he came in on his desk.
The Costello guy who was dying didn't tell because he wanted Leonardo to know that he was also a cop, but you find that out later on the news. Costello was working for himself and keeping the wolves at bay by giving the FBI what they needed from time to time. That was the reason that he hadn't been in jail. Watch it again. Excellent movie, lots of intrigue.