Since the defilement of the beard, particularly with saliva, was a mark of shame and dishonour (see Deuteronomy 25:9; Job 17:6; Job 30:10), no sane Israelite man would defile their beard intentionally. It is to David’s shame that he behaved in this manner and he did so in an attempt to save his life. It is possible that strange/mad behaviour was regarded with superstition at the time. With such individuals being perceived as a danger to society,1 David was hopeful that he would have been sent away. The idea of acting like a madman is used elsewhere in Scripture to describe behaviour that results from being drunk (Jeremiah 25:16; Jeremiah 51:7), terrified (Jeremiah 50:38), or reckless (Jeremiah 46:9; Nahum 2:4).2
13 So he changed his behavior before them and pretended to be insane in their hands and made marks on the doors of the gate and let his spittle run down his beard.