"You shall not take revenge"Leviticus 19:18.

We are forbidden to avenge one bad deed with another. A typical example of the revenge that the Torah prohibits: David asks Isaac to lend him his scythe, and Isaac refuses. Next day, Isaac needs an axe, and asks David whether he can borrow his. "I will not lend you my axe," David responds, "just as you didn't lend me your scythe..."