Trivia: This was Martin Scorsese's first film to win Best Picture. And his first Oscar for best Director. Martin also said that this was the first movie he ever made with a plot.
Trivia: The title track to the movie, which plays at the menu and twice in the movie, is "I'm Shipping Up to Boston", a hit song by the Dropkick Murphys. The Dropkicks are a Boston-based Irish band, fitting for a movie about Irish mobsters in Boston. The song is also the entrance music for Red Sox closer Jonathon Papelbon.
Answer: The assumption is that Wahlberg learned of Damon's betrayal from the envelope given to Vera Farmiga, Damon's therapist girlfriend. It could also have been for revenge, but Wahlberg did know that Damon was the rat when he killed him.
Dandude