Have I need of madmen?.... Or fools, do I want them? have not I enough of them already at my court? I want wise men, and not fools and madmen. The Jews say that the wife and daughter of Achish were mad; that while David was playing the fool and madman without, they were acting the same part within; so that Achish had enough of that sort of diversion, if it was to be reckoned such; as it was according to the taste of some persons, who used, as in later so in earlier times, to keep fools in their houses to make them sport; but Achish had enough of that, and too much, at least needed no more:that ye have brought this [fellow] to play the madman in my presence?
or act the part of a fool before me:shall this [fellow] come into mine house? court or palace, and have a post there; you need be in no pain about it; he is neither agreeable to me, nor fit for any; and therefore Achish drove him away as a fool or madman, instead of imprisoning him as an enemy, or taking away his life; see the title of the thirty fourth psalm, Ps 34:1.
Midrash Tillim apud Abarbinel. in loc.