In The Departed, Billy Costigan he was very intelligent and could have been anything he wanted to be, but he chose to become a state trooper. Why? What was his goal for becoming a cop and going undercover?
Emanuel Levy has some relevant quotes from Leonardo DiCaprio:
Billy Costigan came up from the streets of Boston, and DiCaprio remarks that his character's motivation to become a police officer is rooted in his desire to escape his upbringing: Billy comes from an underworld background and has all the chips stacked against him in a lot of ways. I think he joins the police because he has no other options, and he wants to do things differently than his family did.”
“Ironically, he is asked to go undercover and pretend to be the very thing he was determined not to become. At the heart of it, I think Billy is ultimately trying to redeem himself and not just be a product of his environment, but he ends up deep in a situation that is extremely dangerous and deceitful. There are moments when he could so easily be caughtall the arrows are pointing in his direction as the 'rat,' and everything begins to cave in around him.”
The same quote is also in "The art of writing and making films: THE DEPARTED".