question about The Departed ending scene

[iambusto]

WARNING ************** WARNING **********
If you havent seen the movie, The Departed, please skip the post and go watch the movie instead.
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anyways, those who have seen this movie, I have a question. You guys saw the rat in the very last scene by the window, just when Mark Wahlberg finishes off Matt Damon. Was the rat just symbolic for the whole film, or was Mark Wahlberg the "3rd rat".

We know that Jack Nicholson put 2 rats in the police department, even though Matt damon thought he was the only rat. Could it be that both the rats didnt know that there was a 3rd rat....aka Mark Wahlberg.

Just food for thought. what do you guys think ?