Albert Barnes Commentary Proverbs 21:18

"The wicked is a ransom for the righteous; And the treacherous [cometh] in the stead of the upright." — Proverbs 21:18 (ASV)

Compare the marginal reference. Evildoers seem to draw down the wrath of God upon their heads, and so become, as it were, the scapegoats of the comparatively righteous.