This is the kjv bible states that scripture:
And the evilspirit from the LORD was upon Saul, as he sat 6 in his house with his javelin in his hand: and Davidplayed 18 with [his] hand.
Spirit can also be defined as this: (Hebrew word for spirit in this instance being "ruwach"
air, anger, blast, breath, X cool, courage, mind, X quarter, X side, spirit((-ual)), tempest, X vain, ((whirl-))wind(-y).
Seems to me that God was just angry at Saul because of his attitide. Notice the beginning of the next verse that states Saul's attitude:
And Saulsought 17 to smite 53 David
And even further then that, didn't God protect David and Saul after this event occured?