Who was Mark Wahlberg's character really?

Rating: 8/10 (13 October 2006)

In the end, when Mark Wahlberg's character (Dignam) shot Damon's character (Sullivan), did that prove that Wahlberg was also a mole just covering his tracks, or was it revenge for Damon being a cop killer? If Wahlberg was just getting revenge on Damon, the girlfriend must have given Walhberg the yellow envelope that Leo gave her. But if Walhberg was a good guy, wouldn't he just hand it over to the police? Why would he go and kill Damon and not let the truth out? If Walhberg was also a mole, he had to kill Damon to cover his tracks. I think that it makes more sense that he was a mole... or a rat, as the film cleverly portrays with the big rat crawling on the porch. They are pretty crafty in Hollywood.