L.A. Confidential (1997) is an excellent illustration of the need to fight fire with fire.

Rating: 10/10 (25 November 2001)

WARNING - POSSIBLE SPOILERS HEREIN.
This movie portrays matrixes within matrixes within matrixes, or games within games within games, etc. and illustrates the intense efforts necessary to arrive at the center of the maze.
Deception, corruption, and dishonesty are everywhere but fortunately there are some who will go to any length to undue the ball of string.
Once exposed, it takes evil to fight evil, with care taken to not become evil in the process. To quote Christopher Lambert in Beowulf, "The only thing that keeps me from becoming evil is fighting evil" and Nicolas Cage in re 8MM, "One needs to be careful when fighting monsters that they don't become a monster".
This movie portrays in an exceptional manner the forces than can be behind crime and murder and those forces' quests for power and greed. Additionally, it takes one from the nieve state of mind of the young cop (Guy Pearce) to the reality of the world we live in; a maturation process. It also illustrates the driving forces behind human behaviour, i.e., Russell Crowe's character's war against men who abuse women and how his fear and loathing of those characteristics results in his becoming what he feared and loathed in one scene.
Overall, an exceptional and informative movie about human nature with as good an ending as can be expected under the circumstances.