John Gill Commentary 1 Samuel 18:10

"And it came to pass on the morrow, that an evil spirit from God came mightily upon Saul, and he prophesied in the midst of the house: and David played with his hand, as he did day by day. And Saul had his spear in his hand;" — 1 Samuel 18:10 (ASV)

And it came to pass on the morrow
After the women had met him with their music and dancing, and when returned home:

that the evil spirit from God came upon Saul ;
thinking on the above things that had passed, he became melancholy:

and he prophesied in the midst of the house ;
either really, delivering out divine songs, as the prophets did; according to Abarbinel, he foretold that David would be king, and the kingdom would be taken from him, and given to him; or he feigned himself a prophet, mimicking their motions and gestures; or, as the Targum, acted like a mad man, or a fool, uttering foolish words, and using ridiculous gestures, which seems most agreeable to the evil spirit in him:

and David played with his hand as at other times ;
upon his harp, to remove the evil spirit, or melancholy disposition from Saul.

For though he was now advanced at court, and an officer in the army, and high in the affections and applause of the people, yet he did not think it below him to act as a musician, to do service to his prince. Of such an humble, kind, and ingenuous disposition was he:

and [there was] a javelin in Saul's hand ;
a kind of spear, or half pike, which he had taken into his hand on purpose to kill David while playing; for persons in such circumstances as his, as they are very mischievous, so very subtle at contriving.