Playing With: If You Kill Him You Will Be Just Like Him
Basic Trope: After fighting his way to the final villain, the hero lets the villain go to avoid the guilt of killing him.
Straight: After Keith, the villain, killed several people, Bob, the hero, fights his way to Keith. When Bob has Keith at his mercy, he lets Keith go, after Alice, his girlfriend, begs Bob not to "sink to Keith's level".
Keith killed his victims as an act of revenge against his victims, who also killed as another act of revenge. If Bob were to kill Keith, the Cycle of Revenge would continue, and Bob would be yet another link in the bloody chain.
Deconstructed: Keith continues his reign of terror, and when Bob and Alice return home, instead of being welcomed as heroes, they are demonized, condemned, spat upon, called disgusting words, and later are tried and executed for dereliction of duty and extreme negligence.
Reconstructed: Keith is moved by Bob's gesture of mercy and reforms.