For they have spread out. In this verse David shows that the ungodly had deserved the awful ruin which he predicted would befall them, since they had not only molested mortal man, but had also rushed forth in the fury of their pride to make war against God himself. No man, as has been stated in our exposition of the second psalm, could offer violence to the kingdom of Israel, which was consecrated in the person of David, by the commandment of God, without making foul and impious war against God. Much more when persons directly attack the kingdom of Christ to overthrow it, is the majesty of God violated, since it is the will of God to reign in the world only by the hand of his Son. As the Hebrew word nth, natah, which we have translated to spread out, also sometimes signifies to turn aside, it may not unsuitably be here rendered either way. According to the first view the meaning is, that the wicked, as if they had spread out their nets, endeavored to subject to themselves the power of God. According to the second the meaning is, that for the purpose of hindering, and as it were swallowing up his power, they turned aside their malice, so as to make it bear against it, just like a man who, having dug a great ditch, turned aside the course of some torrent to make it fall within it. The Psalmist next declares, that they devised a stratagem, or device, which would fail of its accomplishment. By these words he rebukes the foolish arrogance of those who, by making war against God, manifest a recklessness and an audacity which will undertake any thing, however daring.

For they intended evil against thee - literally, "They stretched out evil." The idea seems to be derived from "stretching out" or laying snares, nets, or gins, for the purpose of taking wild beasts. That is, they formed a plan or purpose to bring evil upon God and his cause: as the hunter or fowler forms a purpose or plan to take wild beasts or fowls. It is not merely a purpose in the head, as our word "intended" would seem to imply; it supposes that arrangements had been entered into, or that a scheme had been formed to injure the cause of God - that is, through the person referred to in the psalm. The purposes of wicked men against religion are usually much more than a mere "intention." The intention is accompanied with a scheme or plan in their own mind by which the act may be accomplished. The evil here referred to was that of resisting or overpowering him who was engaged in the cause of God, or whom God had appointed to administer his laws.
They imagined a mischievous device - They thought, or they purposed. The word rendered "mischievous device" מזמה mezimmâh - means properly "counsel, purpose; then prudence, sagacity;" then, in a bad sense, "machination, device, trick." Gesenius, Lexicon. Proverbs 12:2; Proverbs 14:17; Proverbs 24:8.
Which they are not able to perform - literally, "they could not;" that is, they had not the power to accomplish it, or to carry out their purpose. Their purpose was plain; their guilt was therefore clear; but they were prevented from executing their design. Many such designs are kept from being carried into execution for the want of power. If all the devices and the desires of the wicked were accomplished, righteousness would soon cease in the earth, religion and virtue would come to an end, and even God would cease to occupy the throne.

For they intended evil - Sinners shall not be permitted to do all that is in their power against the godly; much less shall they be able to perform all that they wish.

For they (g) intended evil against thee: they imagined a mischievous device, [which] they are not able [to perform].
(g) They laid as it were their nets to make God's power bend to their wicked enterprises.

For they intended evil against thee,.... All evil, whether in thought or deed, if not immediately and directly, yet is ultimately against the Lord, whose law is transgressed, and who is despised and reflected upon as a lawgiver; all sin is an hostility committed against God, or against Christ, against the Lord and his Anointed, or against his people, who are all one as himself: the intention of evil is evil, and is cognizable by the Lord, and punishable by him:
they imagined a mischievous device, which they are not able to perform; not the death of Christ; that was indeed in itself a mischievous device of theirs, but that they performed, though they had not their end in it; they expected his name would then perish, and they should hear no more of him: but rather it respects his resurrection from the dead, they could not prevent, though they took all imaginable care that them might be no show of it; and when they found he was really raised from the dead, they contrived a wicked scheme to stop the credit of it, but in vain, Matthew 27:63; and Jews and Gentiles, and Papists, have formed schemes and done all they can to root the Gospel, cause, and interest of Christ, out of the world, but have not been able to perform it.

This terrible overthrow, reaching to posterity, is due to their crimes (Exodus 20:5-6).

(Hebrews.: 21:12-13) And this fate is the merited frustration of their evil project. The construction of the sentences in Psalm 21:12 is like Psalm 27:10; Psalm 119:83; Ew. 362, b. נטה רעה is not to be understood according to the phrase נטה רשׁת (= פּרשׁ), for this phrase is not actually found; we have rather, with Hitzig, to compare Psalm 55:4, 2-Samuel 15:14 : to incline evil down upon any one is equivalent to: to put it over him, so that it may fall in upon him. נטה signifies "to extend lengthwise," to unfold, but also to bend by drawing tight. שׁית שׁכם to make into a back, i.e., to make them into such as turn the back to you, is a more choice expression than נתן ערף, Psalm 18:41, cf. 1-Samuel 10:9; the half segolate form שׁכם, (= שׁכם) becomes here, in pause, the full segolate form שׁכם. חצּים must be supplied as the object to תּכונן, as it is in other instances after הורה, השׁליך, ידה; כּונן חץ, Psalm 11:2, cf. Psalm 7:14, signifies to set the swift arrow upon the bow-string (מיתר = יתר) = to aim. The arrows hit the front of the enemy, as the pursuer overtakes them.

Thee - Against God, not directly, but by consequence, because it was against David, whom God had anointed, and against the Lord's people, whose injuries God takes as done to himself.