Proverbs 24:28-29 (revenge)

Do not testify against your neighbor without cause— would you use your lips to mislead? Do not say, “I’ll do to them as they have done to me; I’ll pay them back for what they did.” (NIV)

Don’t talk about your neighbors behind their backs— no slander or gossip, please. Don’t say to anyone, “I’ll get back at you for what you did to me. I’ll make you pay for what you did!” (The Message)