Charles Ellicott 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)

The wicked shall be a ransom for the righteous. —The righteous is delivered out of trouble (Proverbs 11:8), and the wicked comes in his place to receive upon his own head God’s descending punishment. So it was with Mordecai and Haman.