When king Saul rebelled against God, God judged Him by rejecting him from being king of Israel (1 Sam. 15:23). After Samuel anointed his successor, David, the Holy Spirit came upon David from that day onward (16:13). By contrast, “the Spirit of the LORD departed from Saul, and an evil spirit from the LORD troubled him” (16:14).
Was the spirit from God that tormented Saul an unholy spirit or was he an angel who was sent by God to distress Saul? Some believers are troubled to think that this spirit was actually an evil spirit in the sense of being a demon. For them, for God to use such a spirit creates theological problems with their view of God and His separateness from sin.
An examination of many similar Scripture passages helps to answer the question of the identity of the spirit that tormented Saul.
1. Adam and Eve were tempted by Satan, who could only have assaulted them had God permitted him to do so (see point 2 for Scriptural support for this interpretation):
2Co 11:3 But I fear, lest by any means, as the serpent beguiled Eve through his subtilty, so your minds should be corrupted from the simplicity that is in Christ.
Rev 20:2 And he laid hold on the dragon, that old serpent, which is the Devil, and Satan, and bound him a thousand years.
2. Job was assaulted by Satan on more than one occasion when God gave him permission to do so:
Job 1:12 And the LORD said unto Satan, Behold, all that he hath is in thy power; only upon himself put not forth thine hand. So Satan went forth from the presence of the LORD.
Job 2:6 And the LORD said unto Satan, Behold, he is in thine hand; but save his life.
3. Because of his sinfulness, God judged king Ahab through a lying spirit:
2Ch 18:18 Again he said, Therefore hear the word of the LORD; I saw the LORD sitting upon his throne, and all the host of heaven standing on his right hand and on his left.
19 And the LORD said, Who shall entice Ahab king of Israel, that he may go up and fall at Ramothgilead? And one spake saying after this manner, and another saying after that manner.
20 Then there came out a spirit, and stood before the LORD, and said, I will entice him. And the LORD said unto him, Wherewith?
21 And he said, I will go out, and be a lying spirit in the mouth of all his prophets. And the LORD said, Thou shalt entice him, and thou shalt also prevail: go out, and do even so.
22 Now therefore, behold, the LORD hath put a lying spirit in the mouth of these thy prophets, and the LORD hath spoken evil against thee.
4. Paul’s affliction at the hands of Satan was divinely given him:
2Co 12:7 And lest I should be exalted above measure through the abundance of the revelations, there was given to me a thorn in the flesh, the messenger of Satan to buffet me, lest I should be exalted above measure.
The use of the divine passive (“was given”) shows that God was the One who allowed Paul to be afflicted by Satan.
5. God will judge many people in the future who will have rejected His truth by sending strong delusion upon them, which will be the work of evil spirits:
2Th 2:8 And then shall that Wicked be revealed, whom the Lord shall consume with the spirit of his mouth, and shall destroy with the brightness of his coming:
9 Even him, whose coming is after the working of Satan with all power and signs and lying wonders,
10 And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved.
11 And for this cause God shall send them strong delusion, that they should believe a lie:
12 That they all might be damned who believed not the truth, but had pleasure in unrighteousness.
These five passages provide ample biblical support for holding that king Saul was tormented by an unholy spirit from God and not just a “distressing spirit” (1 Sam. 16:14 in the NKJV). In addition, the Spirit’s departure from Saul prior to the evil spirit’s coming upon him also points to his being an unholy spirit that came to torment Saul once the Holy Spirit was no longer upon him (cf. 1 Sam. 10:6).
 Additionally, the identification of this spirit as an evil spirit versus a distressing spirit has vital bearing on determining the moral character of the instrumental music that David played for Saul (see my post Correcting a Wrong Handling of the Accounts of David’s Music Ministry to Saul).