why frank costello get killed, even though he just a FBI Informer,
how sergeant diginam know colin sulivan is rat in police department?

I think the film LITERALLY shows why that was done or why this person knows something. I'm going to try my fucking best to not sound like an asshole and shit on your goals to knowledge. But... When you watch these films. In what country do you watch them, are they dubbed and/or subtitled, and how strong is your grasp of the English language?

All you need to know is there is literally a rat crawling across the balcony. THat's why.

Teh end.

"The rat symbolizes obviousness."

Subtly isn't Scorsese's strongest suit.

It's all clear if you watch Infernal Affairs.

At the very least, it's shorter and less profane.

This has all the answers and contains spoilers :

<iframe width="504" height="378" src="//www.youtube.com/embed/AgSja633_F4" frameborder="0" allowfullscreen></iframe>

That was actually pretty interesting.

I remember that theory. It does seem to hold water and is far more interesting than the mystery of the tacked-on ending.

fuck. i wanted to see this. spoiler tags!

Wow. That actually makes sense. Uhhhhh... Has anyone asked Scorsese or Damon about it?

Damn.

I never picked up on the subtle moments (the real estate agent, the dinner with Madeline) but I always theorized that his impotence was a sign that he was actually deep in the closet and that it fit in with the theme of the movie. Now after seeing those subtle moments it makes a whole lot more sense.

please give relevant answer's to my question,
please what i should i do, i am new user, that's why i am not getting relevant answer's to my question?

People have given you a valid answer. I just think you're having difficulty understanding it because of the obvious language barrier.

The reason you're not getting relevant answers to your questions is because what you're asking is very obviously shown to you in the film.

Costello gets killed BECAUSE he's an FBI informant.

Dignam knows Sullivan is the rat because Costigan gave Madeline the evidence to give to Dignam.

fixed.

what do u mean by fixed?

so why cant help me from language barrier, instead of telling me it's a language barrier.

you just repeated my questions, i am asking if he is FBI informant why he get killed,
& also there is no scene show "Madeline giving evidence to Dignam", so how diginam know sullivan is rat?

Because none of us speak your language. The best we can do is tell you that DiCaprio gave Farmiga the evidence that Damon was a mole and Farmiga passed the evidence to Whalberg. Whalberg then killed Damon.

I dunno, seems like a stretch. The first time I saw the movie, I thought the real estate agent was hitting on Sullivan. After a couple viewings, my guess is the real estate agent is implying that a state police detective doesn't have the money to pay the monthly rent / mortgage. It's about classism, not sexuality. The rest of the "hints" are simply about the impotency issue. That's how I've always taken all of that.

in what way, ur reply is correct answer to my question,
please please never give irrelevant answer to my question, if dont know the answer please skip my post,asnwer the other post
that's all i can tell.

no problem, why cant u skip my post, instead of giving reply if u dont understand my question.

i lack respect because i am new user?

they killed the FBI informant because they did not want him informing the FBI

No, that's not why. I'll give your grammar issues a pass because it's evident English isn't your first language. Your rudeness however, is where this lack of respect comes from.