Ending of The Departed

Why did Mark Wahlberg kill Matt Damon at the end? At first, I just assumed it was because he didn't like Matt Damon, and he found out he was the rat, which made him hate him more. Now, I'm wondering: Were they actually implying that Mark Wahlberg was another one of Costello's men?

Does anyone know?

I definitely don't think Wahlberg was one of Costello's men. I think he knew or at least suspected that Damon's character was responsible for Captain Queenan and Billy Costigan's deaths and that it was a vengeance killing.

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Haha the good old Departed ending. Wahlberg was not one of Costello's men. He was actually a good cop that lost a lot because of Damon. The reason he shoots Damon was because he found out he was the rat so he killed him in revenge.

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The package that DiCaprio gave the therapist contained evidence against him. This package proved that he was a rat, so Wahlberg killed him.

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"A laugh can be a very powerful thing. Why, sometimes in life, it's the only weapon we have."

Suspect's Reviews

I agree with this one. The package Dicaprio left the therapist (to open if he should die) contained the taped convo between Damon the rat and boss jack Nicholson that he had already sent Damon . Walbergs character was taking revenge as I assume there would be instructions for him to be contacted and provided the evidence. Walberg was no rat Imo.

This is one of the best movies i ever watched. I think it's because he wants to prove that he's the best and that he's a man of honor. He doesn't want to be a man of honor and he doesn't want to be the best. That's the real reason why he kills Matt Damon. I saw this film with home theater in my friend's house. If you have such opportunity, use it. I want one of these (https://www.bestadvisor.com/home-theater-systems ) systems to my home

The thing I never understood about the ending is that on the roof, DiCaprio asks the Anthony Anderson cop "Where's Dignam, I told you to bring Dignam". So if Walhberg (Dignam), was given the evidence on Damon before, why wouldn't he go there with the Anderson cop then? Did Dignam just not feel like it that day and didn't want to go? Why not? Isn't that important and catching the rat was important to him? This is never explained why he didn't go.