And the evil spirit from the Lord was upon Saul
His melancholy and frantic disorder returned upon those victories of David, and he grew envious, jealous, spiteful, and malicious:
as he sat in his house with his javelin his hand;
which either describes the posture he was in when the evil spirit came upon him; or the effects of it, he became dull and melancholy, did not care to go abroad, but kept at home, and was suspicious of everybody; and therefore kept a javelin in his hand to defend himself; or it may be rather to dispatch David with it, when an opportunity should offer, which quickly did:
and David played with [his] hand;
on some instrument of music, particularly the harp, to drive away the evil spirit, the melancholy disorder, from Saul; which showed his humility, that though he was an officer in the army, had a considerable post in it, yet deigned to act the part of a musician to Saul, and his great kindness and affection for him his sovereign, willing to serve him what he could to promote his health and comfort, and the trust and confidence he put in his promise and oath, or rather in the providence of God for his protection in the way of his duty, though he knew how spiteful and injurious Saul had been to him.