C. F. Keil and Franz Delitzsch on Proverbs 21:15

Pro 21:15 15 It is a joy to the just to do justice,      And a terror for them that work iniquity.

To act according to the law of rectitude is to these as unto death; injustice has become to them a second nature, so that their heart strives against rectitude of conduct; it also enters to little into their plan of life, and their economy, that they are afraid of ruining themselves thereby. So we believe, with Hitzig, Elster, Zöckler, and Luther, this must be explained in accordance with our interpretation of Pro 10:29. Fleischer and others supplement the second parallel member from the first: וּפעל און מחתּה לפעלי אין; others render 15b as an independent sentence: ruin falls on those who act wickedly. But that ellipsis is hard and scarcely possible; but in general מחתה, as contrasted correlate to שׂמחה, can scarcely have the pure objective sense of ruin or destruction. It must mean a revolution in the heart. Right-doing is to the righteous a pleasure (cf. Pro 10:23); and for those who have און, and are devoid of moral worth, and thus simply immoral as to the aim and sphere of their conduct, right-doing is something which alarms them: when they act in conformity with what is right, they do so after an external impulse only against their will, as if it were death to them.