John Calvin Commentary Psalms 140:9

"As for the head of those that compass me about, Let the mischief of their own lips cover them." — Psalms 140:9 (ASV)

As for the head, etc. There may be a doubt whether, under the term head, he refers to the chief of the faction opposed to him, for we can suppose an inversion in the sentence and a change of the plural to the singular number, bringing out this sense.

Let the mischief of their wicked speeches, which they intended against me, fall upon their own head. As almost all interpreters, however, have taken the other view, I have adopted it, only understanding the reference as being to Saul rather than Doeg. There follows an imprecation upon the whole company of his enemies generally, that coals may fall upon them, alluding to the awful fate of Sodom and Gomorrah.

We find this elsewhere (Psalms 11:6) set forth by the Spirit of God as an example of Divine vengeance, to terrify the wicked; and Jude (Jude 1:7) declares that God testified, by this example of everlasting significance, that He would be the Judge of all the ungodly.

Some translate what follows — the wilt cast them into the fire, which could be acceptable. But as ב (beth) in Hebrew often denotes instrumentality, we may properly render the words — thou wilt cast them down By fire, or With fire, as God sent it against Sodom and Gomorrah.

He prays that they may be sunk into deep pits, from where they may never rise. God sometimes heals those whom He has struck with great severity; David cuts off the reprobate from the hope of pardon, knowing them to be beyond recovery. If they had been disposed to repentance, he would have been inclined on his part to mercy.