Why Does God Send an Evil Spirit on Saul?

QUESTION:  In 1 Samuel 19:9, an evil spirit is said to come from the Lord.  One translation says, “of the Lord”.  How can a good God have or send an evil spirit?  Or is the Message translation right in calling this a “bad mood” from God?
RESPONSE:  No, the most natural reading is that Saul is tormented by an outside spiritual agent.  And this agent is evil – a demonic spirit.  So the problem as you noted, is this impression that God is seeming to act in demonic or evil ways.
1 Sam 19:9 is not the only time the writer of Samuel uses language that seems to indicate God is a source of evil.  Also, in 1 Sam 16:14 another time an evil spirit “from the Lord” came on Saul.  And in 2 Sam 24:1 it says that God’s anger was incited against Israel and caused David to sin in the census he took.
I want to focus on that last event because it’ll help us understand the others.  Interestingly, there’s a parallel account of the census in 1 Chron 21:1, and there, the author explicitly notes that it was Satan – not God – that incited David to sin.  And that dichotomy looks like it creates a whole new problem of a contradictory account.  But actually it helps us understand how the ancient Bible authors looked at causation.  And the way they looked at it, was very fluidly through the lens of the supremacy of Yahweh.
When it comes to causes, we know at least three kinds:  necessary, conditional and contributory.  A necessary cause is required for a certain effect, like the temperature being below 32 degrees is NECESSARY to cause water to freeze.  A conditional cause is needed, but not absolutely, like sex is a needed CONDITION to cause pregnancy, but other conditions are needed too, like ovulation etc.  Contributory causes may CONTRIBUTE to an effect, but alone they do not cause it, like not smoking is not a cause of long life, but may contribute to it.
So when it comes to God, the authors had no problem smashing together all these kinds of causes.  The reason is that they looked at the Lord’s permission as the conditional cause for any effect.  If something happens, Yahweh must have permitted it – even if other conditions are needed too.  Now, if they were to explain all causes, they’d say that nothing happens without God, but some effects, like evil effects, must have other conditions in order to come about.  Why?  Because the Bible writers are absolutely unbending on the idea that God is completely righteous (Judg 5:11; Ezra 9:15; Ps 7:9; Rev 16:5), hates evil (Zech 8:17), and never does anything unjust (Rom 9:14).
However, the authors don’t take time to tease out all this theology in every narrative.  Thus, their very high view of the Lord’s power as a conditional cause  for everything, meant that their language could slip into God seeming to do everything, including evil things, like in 2 Sam 24:1, causing David to sin. 
But as we see, another inspired author (of Chronicles) looks at the same event and knows that God’s permission was only a conditional cause, the actual cause of the sin was Satan, for they knew that “God tempts no man” (James 1:13).  Of course, even Satan, technically is only a conditional cause, for the final or necessary cause of sin, requires an actual sinner!  David.  Sin can’t be sin, unless you have one who commits it freely, even if other causes are needed conditions.  In this case, David is the necessary cause of the sin, not God.
So, back to Saul and the evil spirit “sent by the LORD”.  Here, the author distances God’s causation of Saul’s torment one step, in that he makes clear God is not the one actually causing the torment.  Who is?  The evil spirit is.  The question then is this: is God being unrighteous if he sends an evil spirit to accomplish some task?  Only, if the task itself, which God has permitted, is unrighteous.  But is it wrong for God to use an evil spirit to achieve a good end?  No.

Saul had lived a life of chronic disobedience to God, and therefore had opened himself to demonic oppression.  That was Saul’s choice.  And it was God’s just punishment, because of Saul’s disobedience, to torment him, using a spirit’s free choice to do so.  So three wills are choosing freely in this scenario:  God is choosing freely to discipline a wayward King, an evil spirit is choosing freely to torment a human (God is not forcing the spirit to do that – it’s an evil spirit after all), and Saul is choosing freely to be wayward.

But just because this is true, doesn’t mean that God’s end is punishment alone.  We know that Paul “hands people over to Satan” for the ultimate end that disobedient teachers or believers are “taught not to blaspheme” and to be “saved” (1 Tim 1:20; 1 Cor 5:5).  So we can rightly surmise that God’s ultimate design in using that evil spirit (doing what it wanted to do) was to drive Saul to repentance.
The evil spirit does not know it’s being used for a good end.  We humans also can freely do evil, which God uses without our knowledge for some good end (Gen 50:20).  Evil spirits are no different than humans, as far as that goes.  God is not doing the evil, but he is taking the freely chosen evils and in his grand design, orchestrating or using them for good.  He’s that awesome.

So the spirit is a dog on a chain doing what dogs do, bite.  And God is at the end of the leash, doing what God does, orchestrating good ends.  God himself doesn’t “bite” (that is, sin), but in a fallen world, God uses sin he doesn’t necessarily cause for redemptive purposes.