Thou shalt destroy - Thou wilt bring to ruin; thou wilt cause to perish; that is, cause to perish as the wicked are caused to perish, by being punished. The idea is that God could not approve their cause; could not favor them; could not give them prosperity, and that they must be overthrown and punished. As in the previous verses, so here, David refers to this as a general characteristic of God, but with an implied reference to his enemies.
Them that speak leasing - Lies; the word "leasing" being the old Saxon word to denote falsehood. See Psalm 4:2. It is not found elsewhere in our common version. The allusion here is to his enemies, and the idea is that they were false and treacherous; a description which will well apply to them on the supposition that this refers to the rebellion of Absalom. See the introduction to the psalm.
The Lord will abhor - Will hate; will hold in abomination. That is, he will show his abhorrence by punishing such as are here referred to.
The bloody and deceitful man - The man of blood and fraud; the man who sheds blood, and is guilty of treachery and fraud. Margin, "man of bloods and deceit." The "man of bloods," - "the plural form being commonly used where there is reference to blood-guiltiness or murder." - "Prof. Alexander." See Genesis 4:10; Psalm 51:14. The idea seems to be that of shedding "much" blood. The reference here, as before, is to a general characteristic of the Divine Mind, with a special reference to the character of David's enemies, as being distinguished for fraud and blood-guiltiness. On the supposition (see introduction) that this refers to the rebellion of Absalom, there can be no difficulty in seeing the propriety of the application. It was on these grounds that the psalmist directed his prayer to God. He was confident that his was a righteous cause; he was as sure that his enemies were engaged in a wicked cause; and he felt, therefore, that "he" might go before God and seek his interposition, with the assurance that all his attributes, as a righteous and holy God, would be enlisted in his favor. God has "no" attribute which can take part with a sinner, or on which a sinner can rely; the righteous can appeal to "every" attribute in the divine nature as a ground of confidence and hope.

That speak leasing - Falsity, from the Anglo-Saxon leasunge, a lie, falsity, deceit; from leas, lie, which is from the verb leasian to lie. See on Psalm 4:2 (note).
The Lord will abhor the bloody and deceitful man - איש דמים ish damim, the man of bloods; for he who has the spirit of a murderer, will rarely end with one bloodshedding. So the Jews, who clamored for the blood of our Lord, added to that, as far and as long as they could, the blood of his disciples.

Thou shall destroy them that speak leasing,.... Or "a lie" (i); whether in matters of religion; as false doctrines, errors, and heresies, are lies; and so all that deny the deity, sonship, and Messiahship of Christ, are liars; and the followers of the man of sin speak lies in hypocrisy: or in common conversation; such are like to Satan, and are abominable in the sight of God; and he will destroy them, either with a temporal destruction, with bodily diseases, as Gehazi was smitten with a leprosy; and with corporeal death, as Ananias and Sapphira; or with eternal destruction, the destruction of body and soul in hell; for all liars have their part in the lake which burns with fire and brimstone;
the Lord will abhor the bloody and deceitful man; or "the man of blood and deceit" (k); that thirsts after blood; who sheds innocent blood, as the Targum paraphrases it. He showed his resentment against Cain, the first of this sort, in a way that was intolerable to him. He very early established a law, requiring that he who shed man's blood, by man should his blood be shed; and he will give the whore of Rome, who has been drunk with the blood of his saints, blood to drink, because she is worthy. And all such who flatter with their tongue, and speak with a double heart, and lie in wait to deceive their neighbour, whether in things temporal or spiritual, are the objects of his abhorrence and indignation; see Psalm 55:23. Now David's enemies being such sort of persons, foolish wicked men, proud and haughty, workers of iniquity, liars, bloody and deceitful men, men that God had an abhorrence of, he therefore hoped and was confident that God would hear his prayers against them, and for himself.
(i) "mendacium", V. L. (k) "vir sanguinum et doli, vel fraudis", Montanus, Cocceius, Gejerus; so Ainsworth.

leasing--a lie.
the bloody . . . man--literally, "man of blood"--murderer.