Against thee, i.e. against God, not directly, but by conseqence, because it was against David, whom God had anointed, and against the Lord's people, whose injuries God takes as done to himself, Malachi 2:8. To perform; such supplements are usual after this verb, as Exodus 8:18 Psalms 101:5, Psalms 139:6 Isaiah 1:13. Or, for which they were unable or insufficient. Or, but they did not prevail, as this verb signifies, Psalms 13:4, Psalms 129:2. This clause seems to be added to teach us this great and necessary lesson, that men are justly punished by God for their wicked intentions, although they be hindered from the execution of them, contrary to what some Jewish doctors and others have taught.
Related Commentaries of Psalms 21:11
Psalms 21:11
11 For they intended evil against thee: they imagined a mischievous device, which they are not able to perform.