Albert Barnes Commentary 1 Samuel 16:15

"And Saul`s servants said unto him, Behold now, an evil spirit from God troubleth thee." — 1 Samuel 16:15 (ASV)

The “evil” or “melancholy” spirit spoken of here was “the Spirit of God,” or “of Jehovah,” in that it was God’s messenger and minister, sent by Him to execute His righteous purpose upon Saul (See 1 Kings 22:19–22).