Charles H. Spurgeon on Psalms 119:78

Let the proud be ashamed. That is, that they may not prosper or succeed in their attempts; for men are ashamed when they are disappointed. All their endeavours for the extirpation of God's people are vain and fruitless, and those things which they have subtilly devised, have not that effect which they propounded unto themselves. "For they dealt perversely with me without a cause." The Septuagint have it aoikwv unjustly. Ainsworth readeth, "With falsehood they have depraved me." It implies two things: first, that they pretended a cause; but, secondly, David avouches his innocency to God; and so, without any guilt of his, they accused, defamed, condemned his actions, as is usual in such cases. When the proud are troublesome and injurious to God's people the saints may boldly commend their cause to God...The Lord may be appealed unto upon a double account; partly, as he is an enemy to the proud, and as a friend to the humble (James 4:6 Psalms 138:6); partly, as he is the portion of the afflicted and oppressed (Psalms 140:12). When Satan stirreth up his instruments to hate those whom the Lord loveth, the Lord will stir up his power to help and defend them. Is not this a revengeful prayer? Answer, No. First. Because those who pray it are seeking their own deliverance, that they may more freely serve God by consequence. Indeed, by God's showing mercy to his people, the pride of wicked ones is suppressed (Psalms 119:134); but mercy is the main object of the prayer.

Secondly. As it concerneth his enemies, he expresses it in mild terms that they may "be ashamed"; that is, disappointed, in their counsels, hopes, machinations, and endeavours. And therefore it is not against the persons of his enemies, but their plots and enterprises. In such cases shame and disappointment may even do them good, They think to bring in the total suppression of God's people, but that would harden them in their sins; therefore God's people desire that he would not let their innocency be trampled upon, but disappoint their adversaries, that the proud may be ashamed in the failing of their attempts.

Thirdly. The prayers of the righteous for the overthrow of the wicked, are a kind of prophecies; so that, in praying, David doth in effect foretell, that such as dealt perversely should soon be ashamed, since a good cause will not always be oppressed: "But he shall appear to your joy, and they shall be ashamed" (Isaiah 66:5).

Fourthly. Saints have a liberty to imprecate vengeance, but such as must be used sparingly and with great caution: "Let them be confounded and consumed that are adversaries to my soul" (Psalms 71:13). Malicious enemies may be expressly prayed against. Thomas Manton.