Why boastest thou thyself in Mischief? - Why dost thou “exult” in that which is wrong? Why dost thou find pleasure in evil rather than in good? Why dost thou seek to triumph in the injury done to others? The reference is to one who prided himself on schemes and projects which tended to injure others; or who congratulated himself on the success which attended his efforts to wrong other people.
O mighty man - DeWette and Luther render this, “tyrant.” The original word would be properly applied to one of rank or distinction; a man of “power” - power derived either from office, from talent, or from wealth. It is a word which is often applied to a hero or warrior: Isaiah 3:2; Eze 39:20; 2 Samuel 17:10; Psalms 33:16; Psalms 120:4; Psalms 127:4; Daniel 11:3; Genesis 6:4; Jeremiah 51:30. So far as the “word” is concerned, it might be applied either to Saul or to any other warrior or man of rank; and Professor Alexander supposes that it refers to Saul himself. The connection, however, seems to require us to understand it of Doeg, and not of Saul, This appears to be clear
(a) from the general character here given to the person referred to, a character not particularly applicable to Saul, but applicable to an informer like Doeg Psalms 52:2-4; and
(b) from the fact that he derived his power, not from his rank and office, as Saul did, but mainly from his wealth Psalms 52:7. This would seem to imply that some other was referred to than Saul.
The goodness of God endureth continually - literally, “all the day.” That is, the wicked man could not hope to prevent the exercise of the divine goodness toward him whom he persecuted, and whom he sought to injure. David means to say that the goodness of God was so great and so constant, that he would protect his true friends from such machinations; or that it, was so unceasing and watchful, that the informer and accuser could not hope to find an interval of time when God would intermit his care, and when, therefore, he might hope for success. Against the goodness of God, the devices of a wicked man to injure the righteous could not ultimately prevail.