C. F. Keil and Franz Delitzsch on Proverbs 21:18

Pro 21:18 18 The godless becometh a ransom for the righteous;      And the faithless cometh into the place of the upright.

The thought is the same as at Pro 11:8. An example of this is, that the same world-commotion which brought the nations round Babylon for its destruction, put an end to Israel's exile: Cyrus, the instrument in God's hands for inflicting punishment on many heathen nations, was Israel's liberator, Isa 43:3. Another example is in the exchange of places by Haman and Mordecai, to which Rashi refers. כּפר is equivalent to λύτρον, ransom; but it properly signifies price of atonement, and generally, means of reconciliation, which covers or atones for the guilt of any one; the poll-tax and “oblations” also, Exo 30:15., Num 31:50, are placed under this point of view, as blotting out guilt: if the righteousness of God obtains satisfaction, it makes its demand against the godless, and lets the righteous go free; or, as the substantival clause 18b expresses, the faithless steps into the place of the upright, for the wrath passes by the latter and falls upon the former. Regarding בּוגד, vid., Pro 2:22. Thus, in contrast to the ישׁר, he is designated, who keeps faith neither with God nor man, and with evil intention enters on deceitful ways - the faithless, the malicious, the assassin.