Infernal Affairs(2004)

A cop goes undercover as a gangster while, simultaneously, a gangster infiltrates the police force, pretending to be a cop. These two sleeper agents live underground for a decade before a series of mistakes clues in all the wrong people as to what's going on and each mole is ordered to root out the double agent--which in both cases happens to be themselves. Torn between conflicting father figures--an avuncular, paranoid gang boss and a morally comprised police superintendent--the two moles slowly feel the net tighten like an icy hand around their necks. Juggling identies, always one mistake from a bullet in their head, and seduced by the attractions of their cover identies, both men slowly unravel, battering themselves bloody against the bars of their own personal hells, while their souls ice over.