8 But yet what is to be done? “Without a cause have they hid for me the corruption of their trap.” What means, “Without a cause”? I have done them no evil, I have hurt them not at all. “Vainly have they reviled my soul.” What is, “Vainly”? Speaking falsely, proving nothing. “Let a trap come upon them which they know not of”. A magnificent retribution, nothing more just! They have hidden a trap that I might know not: let a trap come upon them which they know not of. For I know of their trap.
But what trap is coming upon them? That which they know not of. Let us hear, lest haply he speak of that. “Let a trap come upon them, which they know not of.” Perhaps that is one which they hid for him, that another which shall come upon themselves. Not so: but what? “The wicked shall be holden with the cords of his own sins.” Thereby are they deceived, whereby they would deceive. Thence shall come mischief to them, whence they endeavoured mischief. For it follows, “And let the net which they have hidden catch themselves, and let them fall into their own trap.”
As if any one should prepare a cup of poison for another, and forgetting should drink it up himself: or as if one should dig a pit, that his enemy might fall thereinto in the darkness and himself forgetting what he had dug, should first walk that way, and fall into it....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)