While there are dirty cops in many movies, the dirty cop movie is a genre unto itself. James Ellroy is something of a dirty cop factory, having written L.A. Confidential and the screenplay to Dark Blue, starring Kurt Russell and Ving Rhames, and Rampart, starring Woody Harrelson. In both Confidential and Dark Blue, dirty cops are laws unto themselves, using their authority to punish the wicked regardless of the law, and to enrich themselves. In Ellroy’s world, the dirty cop is the norm. Even his good cops take corruption as a given. As always, it’s a matter of degree.
Both Bad Lieutenants, the former starring Harvey Kietel, and the latter Nicolas Cage, are withering trips through hell that end in cracked redemption.
Antoine Fuqua’s Training Day is among the finest dirty cop movies, featuring a swaggering Denzel Washington that is beyond corrupt. Fuqua and Washington teamed again on the excellent The Equalizer, which could not exist without dirty cops.
Among the lesser known dirty cop movies is Boaz Yakin’s Safe, Jason Statham’s best film, which features Statham as a cashiered, corrupt cop on his last legs who finds meaning in saving the life a Chinese girl savant whom Chinese gangsters use as a numbers cruncher. Safe is also notable for James Hong’s sleaziest performance. The climax involves Statham teaming up with his dirty cop buddies to take down both the Chinese and Russian mobs.
James Mangold’s Cop Land posits a Jersey town consisting almost exclusively of corrupt cops whose efforts to hide their crimes brings local sheriff Sly Stallone in conflict with lifetime corrupt cop player Harvey Keitel.
These are just off the top of my head. You may have others.