Jamieson, Fausset and Brown on 1 Samuel 18:10

10. on the morrow, that the evil spirit from God came upon Saul--This rankling thought brought on a sudden paroxysm of his mental malady.

he prophesied--The term denotes one under the influence either of a good or a bad spirit. In the present it is used to express that Saul was in a frenzy. David, perceiving the symptoms, hastened, by the soothing strains of his harp, to allay the stormy agitation of the royal mind. But before its mollifying influence could be felt, Saul hurled a javelin at the head of the young musician.

there was a javelin in Saul's hand--Had it been followed by a fatal result, the deed would have been considered the act of an irresponsible maniac. It was repeated more than once ineffectually, and Saul became impressed with a dread of David as under the special protection of Providence.

1Sa 18:13-16. Fears Him for His Good Success.