I said in my haste, all men [are] liars. The sin of lying is common to man; there is a natural proneness and propensity to it: men go astray from the womb, speaking lies; yet such who have received the grace of God "put [it] off" with the rest of "the deeds of the old man", and are "children that will not lie".
Wherefore, though the greater part of mankind might deserve this character, yet all and every individual of them did not. However degenerate the age was in which David lived, and the faithful among men were few; yet there were some to whom this imputation did not belong; and therefore, on cool reflection, he owned it was said "in haste"; not with thought and deliberation, but rashly and precipitately, unadvisedly, in a passion, and under a temptation, and when off of his guard; and which he acknowledged and repented of.
The Targum is,"I said in my flight;''when he made haste and fled from Saul, whom he might call a liar and dissembler, pretending respect to him when he had none; and also his courtiers; nay, even Samuel himself, who had anointed him, and assured him he should be king; and yet now he thought he had deceived him, and he should perish by the hand of Saul, and never come to the kingdom, 1Sa 27:1; or when he fled from his son Absalom, whom he might call a liar, who had deceived him with the pretence of a vow; and also Ahithophel and others, who proved treacherous and unfaithful to him.
Some take the words in a quite different sense, as an instance of his great faith; that when he was so greatly afflicted, and obliged to fly, yet declared that every man that should say he should not come to the kingdom was a liar; so Kimchi: and others think his meaning is, that every man is a liar in comparison of God, who is true and faithful to his promises, and not a man, that he should lie.