Charles H. Spurgeon on Psalms 140:9

As for the head of those that compass me about, let the mischief of their own lips cover them. To the Lord who had covered his head amid the din of arms the Psalmist appeals against his foes, that their heads may be covered in quite another sense —covered with the reward of their own malice. David's foes were so many that they hemmed him in, encircling him as hunters do their prey. It is little wonder that he turns to the Lord in his dire need. The poet represents his adversaries as so united as to have but one head; for there is often a unanimity among evil spirits which makes them the more strong and terrible for their vile purposes. The lex talionis, or law of retaliation, often brings down upon violent men the evil which they planned and spoke of for others: their arrows fall upon themselves. When a man's lips vent curses they will probably, like chickens, come home to roost. A stone hurled upward into the air is apt to fall upon the thrower's head. David's words may be read in the future as a prophecy; but in this verse, at any rate, there is no need to do so in order to soften their tone. It is so just that the mischief which men plot and the slander which they speak should recoil upon themselves that every righteous man must desire it: he who does not desire it may wish to be considered humane and Christlike, but the chances are that he has a sneaking agreement with the wicked, or is deficient in a manly sense of right and wrong. When evil men fall into pits which they have digged for the innocent we believe that even the angels are glad; certainly the most gentle and tender of philanthropists, however much they pity the sufferers, must also approve the justice which makes them suffer. We suspect that some of our excessively soft spoken critics only need to be put into David's place, and they would become a vast deal more bitter than he ever was.