Fan Casting Before I Change My Mind in Sort by Tropes

The hero has the villain captured, in his grasp, and there's no way the villain can escape the due justice that's coming to him. However, the villain responds by trying to convince the hero that he (the hero) has to let him (the villain) go free. Cue the hero freeing the villain from his bonds and telling him to leave now "...before I change my mind." Alternately, reverse the scenario: the villain has the hero in his grasp and is ready, willing and able to kill him, but the hero says or does something that somehow convinces the villain not to go through with it.