1 Samuel 21:14

Then said Achish... the man is mad. — The Philistine king would look with peculiar sorrow and repulsion on a madman if, as according to Jewish tradition (see Philippson), his own wife and daughter were insane.

The device, however, succeeded, as David hoped it would, and he was suffered to depart in safety — nay, was even hurried out of the Philistine country. In old times, as now, in many parts of the East, the insane are looked upon as persons in some peculiar way possessed by, and therefore under the more immediate protection of, Deity. The life then of the hunted fugitive was perfectly safe from the moment the Philistines considered him mad.
There is a curious legend in the Talmud in which several events recorded in the Biblical account are confused. Part of it apparently refers to this strange choice of his of Phillstia as a place of refuge. “One day Satan appeared to him (David) in the shape of a gazelle, which, eluding his pursuit, decoyed him into the land of the Philistines. ‘Ah!’ said Ishbi-benob, when he caught sight of him, ‘art thou the man that slew my brother, Goliath?’ So saying, he seized and bound him.” — Treatise Sanhedrin, fol. 95, Colossenses 1:2. The wild legend goes on to explain how, partly by miracle, partly with the aid of Abishai, David slew Ishbi-benob and escaped.