Scorsese understood that in the modern era, institutional loyalty is dead. Billy Costigan wants to be a cop but is treated like a criminal; Colin Sullivan lives a criminal’s life but enjoys the protection of a cop’s salary. The tragedy isn't that they break the law—it’s that they lose their identity trying to serve two masters.
Of course, the dynamic gets even more interesting when the roles corrupt each other. Look at (2006). Here, the gangster (DiCaprio) is actually the cop, and the cop (Damon) is actually the gangster. The anxiety of the genre reaches its peak when you can no longer tell who is wearing the badge and who is holding the gun. the gangster the cop