10 It happened on the next day, that an evil spirit from God came mightily on Saul, and he prophesied in the midst of the house. David played with his hand, as he did day by day. Saul had his spear in his hand;

Matthew Henry's Commentary on 1 Samuel 18:10

Commentary on 1 Samuel 18:6-11

(Read 1 Samuel 18:6-11)

David's troubles not only immediately follow his triumphs, but arise from them; such is the vanity of that which seems greatest in this world. It is a sign that the Spirit of God is departed from men, if, like Saul, they are peevish, envious, suspicious, and ill-natured. Compare David, with his harp in his hand, aiming to serve Saul, and Saul, with his javelin in his hand, aiming to slay David; and observe the sweetness and usefulness of God's persecuted people, and the barbarity of their persecutors. But David's safety must be ascribed to God's providence.