In Rumors Of Another World, Philip Yancey tells a story that illustrates the kind of "revenge" Paul spoke about in Romans 12:20, when he said that showing kindness to an enemy will "heap coals of fire on his head."

When Nelson Mandela became president of South Africa, he appointed a commission to bring to trial those who had been guilty of atrocities during the time of apartheid. Any white officer who voluntarily faced his accusers and confessed his guilt would not be punished.

One day an aged woman was brought face-to-face with the officer who had brutally murdered her only son and her beloved husband. Asked what she wanted from him, she said, "Although I have no family, I still have a lot of love to give." She requested that he visit her regularly so she could mother him. Then she said, "I would like to embrace him so he can know that my forgiveness is real."

Yancey writes that as the elderly woman made her way to the witness stand, the officer became so overwhelmed with shame and remorse that he fainted. The pain that woman inflicted was not sinful revenge but the purifying fire of a God-given love that can lead to repentance and reconciliation. That's redemptive revenge.
  — Herbert Vander Lugt