Q. Is bearing a grudge ever justified?
A. Holding grudges and taking revenge are among the most primal human instincts. These instincts have one positive effect; if everyone knows you will hold them accountable for a slight they will be reluctant to take advantage of you. But it also has a negative effect; since each of us has a powerful tendency to rationalize our actions and view ourselves as in the right, the grudge or act of revenge is likely to be itself interpreted as a slight, and one offensive act, or even one mistakenly interpreted as offensive, can set off a cascade of attacks and counterattacks and perhaps a permanent feud.
In order to prevent this effect, the Torah has a specific prohibition against revenge and grudges: "Don't take vengeance and don't bear a grudge towards the members of My people, and love your fellow as yourself; I am the Lord" (Leviticus 19:18).
The medieval Sefer HaChinuch, a very influential book explaining the scope and rationale of the Torah's commandments, understands this commandment very broadly:
And the substance is that if anyone of Israel harmed or vexed his fellow in any way, and the habit of most people is that they will not desist from seeking out someone who harmed them until they have requited the harmful act or cause [the wrongdoer] sorrow just as he caused sorrow to them. And the Holy One blessed be He prevents us from doing his. . . And the root of this commandment is that a person should know and internalize that anything that happens to him, whether good or bad, its ultimate cause is from God. . . And in addition this commandment has a great benefit to quiet dispute and to remove enmity from people's hearts. (1)
Not to hold a grudge, that is, we are prevented from holding a grudge in our hearts on any harm done us by one of Israel, even if we have agreed not to requite his actions...And the substance of this commandment is like that of revenge.(2)
Transgressions between a man and his fellow are not atoned by the Day of Atonement until [the wrongdoer] reconciles himself with [the victim]. Even if he only taunted him verbally, he needs to reconcile. And if he isn't reconciled the first time, he should go back a second and a third time. . . The forgiver should not be cruel and withhold forgiveness unless he intends the benefit of the wrongdoer. (3)
If he said to him, Lend me your scythe, and he said, No; and the next day he said to him, Lend me your shovel, and he said, I won't lend to you just as you wouldn't lend to me, that is revenge.