Here David says not only that their wicked devices were without success, but that, by the wonderful providence of God, the result was the very opposite of what had been contemplated. He sets this forth in the first place metaphorically, by employing the figure of a pit and a ditch; and then he expresses the same thing in simple terms without figure, declaring, that the mischief intended for others returned upon the head of him who had devised it There is no doubt that it was a common proverb among the Jews, He who hath digged a pit falleth into it; which they quoted when they meant to say, that wicked and crafty men are caught in the snares and traps which they have set for others, or that the contrivers of the ruin of others perish by their own devices. [1] There is a twofold use of this doctrine: the first place, however skilled in craft our enemies may be, and whatever means of doing mischief they may have, we must nevertheless look for the issue which God here promises, that they shall fall by their own sword. And this is not a thing which happens by chance; but God, by the secret direction of his own hand, causes the evil which they intend to bring upon the innocent to return upon their own heads. In the second place, If at any time we are instigated by passion to inflict any injury upon our neighbours, or to commit any wickedness, let us remember this principle of retributive justice, which is often acted upon by the divine government, that those who prepare a pit for others are cast into it themselves; and the effect will be, that every one, in proportion as he would consult his own happiness and welfare, will be careful to restrain himself from doing any injury, even the smallest, to another.

Footnotes

1 - "Tomboyent au mal qu'ils avoyent brasse."--Fr. "Fall into the destruction which they had contrived."

He made a pit - The allusion here is undoubtedly to a method of hunting wild beasts which was common in ancient times. It consists in digging a pit-fall, and covering it over with brush and grass so as to deceive the animals, and then enclosing them and driving them into it. See the notes at Isaiah 24:17.
And digged it - And hollowed it out so as to be large enough to contain his prey, and so deep that he could not escape if he fell into it. The idea is, that the enemy here referred to had laid a secret and artful plan to destroy others. He meant that they should not be aware of his plan until the mischief came suddenly upon them. He was preparing to ruin them, and supposed that he was certain of his prey.
And is fallen into the ditch which he made - Into the pit-fall which he had constructed for others; as if a man who had made a pit-fall for wild beasts had himself fallen into it, and could not extricate himself. That is, he had been snared in his own devices; his cunning had recoiled on himself, and instead of bringing ruin on others he had only managed to bring it on himself. See this sentiment illustrated in the notes at Job 5:13. A remarkable instance of the kind may be found in Esther (Esther. 5-7), in the case of Haman. Indeed, such things are not uncommon in the world, where the cunning and the crafty are involved in the consequences of their own plans, and are taken in meshes from which they cannot free themselves. A straightforward course is easy, and men are safe in it; but it requires more skill than most men are endowed with to manage a crooked and crafty policy safely, or so as to be safe themselves in pursuing such a course. A spider will weave a web for flies with no danger to himself, for he is made for that, and acts as if he understood all the intricacies of his own web, and may move safely over it in every direction; but man was made to accomplish his purposes in an open and upright way, not by fraud and deceit; hence, when he undertakes a tortuous and crooked course - a plan of secret and scheming policy - in order to ruin others, it often becomes unmanageable by his own skill, or is suddenly sprung upon himself. No one can overvalue a straightforward course in its influence on our ultimate happiness; no one can overestimate the guilt and danger of a crooked and secret policy in devising plans of evil.

He made a pit - He determined the destruction of David. He laid his plans with much artifice; he executed them with zeal and diligence; and when he had, as he supposed, the grave of David digged, he fell into it himself! The metaphor is taken from pits dug in the earth, and slightly covered over with reeds &c. so as not to be discerned from the solid ground; but the animal steps on them, the surface breaks, and he falls into the pit and is taken. "All the world agrees to acknowledge the equity of that sentence, which inflicts upon the guilty the punishment intended by them for the innocent." - Horne.

He made a pit and digged it,.... That is, he digged a pit, and made it very large and capacious, to answer his purposes;
and is fallen into the ditch which he made; so it is said of the Heathen, Psalm 9:15; and is exemplified in the case of Haman, who was hanged upon the gallows he had built for Mordecai. Kimchi explains this of Saul's falling upon his own sword, and dying by it, which he drew against David; phrase is proverbial, Proverbs 26:27; the sense of this and the above figurative expressions is literally and properly given in Psalm 7:16.

1-Samuel 18:17; 1-Samuel 31:2 illustrate the statement whether alluded to or not. These verses are expository of Psalm 7:14, showing how the devices of the wicked end in disappointment, falsifying their expectations.