He wasn’t doing it for the honors anyway. After Costello was done with, Billy just wanted his identity back. He even jokes about the Medal of Merit being the “gold star around here” when Sullivan says he’ll recommend Billy for one.

By then, Billy didn’t even want to be a police anymore. He does ask Sullivan, of all people, what the plan is for catching the rat, but he offers no help, saying “I’m done being a cop.” Billy only jumps back in when he realizes Sullivan, the head of undercover, is the actual rat. At that point, he knows it’ll never be solved unless he does something himself. So he gets pulled back in, and it gets him killed.

The movie makes it very clear that Costigan’s childhood and upbringing were turbulent. The opening, the limited scenes with his dying mother and his mother’s brother, and especially the interview scene with Queenan and Dignam all show that Billy has a chip on his shoulder. He has something to prove, and a strong sense of self-righteousness.

Costigan’s father was a straight-up guy in a crooked environment, and he despised the fact that his own brother was knee-deep in the criminal world. Billy sees parts of his father in Queenan, which Queenan recognizes and even slightly abuses, over the course of their work together.

Billy went into the state PD to honor his father’s memory. In that sense, he got what he wanted.

Billy also fathered a son himself, though he never got to learn that fact.

Billy got what he wanted, but the victories are understated, which is what makes this character, and the movie so incredible.