He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you.

Friedrich Nietzsche, Beyond Good and Evil, Aphorism 146

2 Kings 10 (NRSV)

15 After he left there, he came upon Jehonadab son of Recab, who was on his way to meet him. Jehu greeted him and said, "Are you in accord with me, as I am with you?"
      "I am," Jehonadab answered.
      "If so," said Jehu, "give me your hand." So he did, and Jehu helped him up into the chariot. 16 Jehu said, "Come with me and see my zeal for the LORD." Then he had him ride along in his chariot.  17 When Jehu came to Samaria, he killed all who were left there of Ahab's family; he destroyed them, according to the word of the LORD spoken to Elijah.

Romans 12 (NRSV)

9Beloved, never avenge yourselves, but leave room for the wrath of God; for it is written, ‘Vengeance is mine, I will repay, says the Lord.’ 20No, ‘if your enemies are hungry, feed them; if they are thirsty, give them something to drink; for by doing this you will heap burning coals on their heads.’ 21Do not be overcome by evil, but overcome evil with good.