The proud [1] have digged pits for me. He complains that he had been circumvented by the frauds and artifices of his enemies; as if he had said, They have not only endeavored to injure me by open force and the violence of the sword, but have also maliciously sought to destroy me by snares and secret arts. The additional clause, which thing is not according to thy Law, is introduced as an argument, to excite God to exercise his mercy; for he is the more inclined to succor his servants, when he sees that the attempts made upon their welfare involve the violation of his own Law. At the same time, the Psalmist furnishes a proof of his own innocence, intimating that he had deserved no such treatment at their hands, and that whatever they practiced, he, notwithstanding, patiently kept himself under restraint; not attempting any thing which he knew to be contrary to the Divine Law.

Footnotes

1 - "zdym, the proud. The proud here, as well as in many other parts of Scripture, stands for lawless, wicked men. So the rendering of the LXX. Is paranomoi; Vulg. Iniqui. The relative, 'sr, is referred to sychvt, pits, by many persons, as Amyraldus, who thus paraphrases the latter part of the verse: At retia illa, cum lege tua directe pugnant.' Others make zdys the antecedent, of whom they consider the second hemistich as descriptive. The proud, who have not acted according to thy Law, have dug pits for me. The sense is more obvious, according to this latter exposition; for one does not see the force of the phrase, digging pits,' which are not according to God's Law, as if pits might be dug which are according to it." -- Phillips

The proud - Those in high life, or of exalted rank. See the notes at Psalm 119:51.
Have digged pits for me - See the notes at Psalm 7:15. Compare Psalm 35:7; Psalm 57:6; Psalm 94:13.
Which are not after thy law - The word which here refers not to the pits, but to the proud. They who have done this are people who do not regard thy commands; people who are open and public offenders. It is that class of people with whom I have to contend - inert who set at defiance all the laws of God; men high in rank, who wield great power, and who have no regard to the law of God in their conduct. Even they have sought my destruction in the meanest way possible - by covert arts, by underhanded means, by digging pits, as they would for wild beasts.

The proud have digged pits - The Vulgate, Septuagint, Ethiopic, and Arabic, translate this verse thus: "They have recited to me unholy fables, which are not according to thy law." They wish us to receive their system of idolatry, and the tales concerning their gods; but these are not according to thy law. The Anglo-Saxon is the same: They quothed me the unrightwise spells; but no so so law thine.

The proud have (d) digged pits for me, which [are] not after thy law.
(d) They have not only oppressed me violently but also craftily conspired against me.

The proud have digged pits for me,.... Laid snares and temptations in his way, to draw him into sin, and so into mischief; they sought indeed to take away his life, and formed schemes for it. The allusion is to the digging of pits for the taking of wild beasts; which shows the ill opinion they had of David, and their ill usage of him; see Psalm 7:15;
which are not after thy law; no, contrary to it; which forbids the digging of a pit, and leaving it uncovered, so that a neighbour's beast might fall into it, Exodus 21:33; and if those might not be dug to the injury of beasts, then much less to the injury of men, to the hurt of the servants of the Lord, or to the shedding of innocent blood, which the law forbids.

pits--plots for my destruction.
which--rather, "who," that is, "the proud"; "pits" is not the antecedent.

Who - Who have no respect to thy law.