Thirukkural-On Virtue-Not Causing Injury-Kural 313

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A vengeful harm done even on an unprovoked wrong, Will bring endless evil in its trail.

When a man has not done any harm to another and still the latter has injured him unprovoked, it is the code of the ascetic, or in fact of any righteous man, not to return evil. Such an act of revenge will entail for the ascetic the loss of all the benefits of his penance and for the righteous man, all his virtues, and steep them in sin without redemption.