Jaregala,

This is a very interesting question.

What I believe is in the character of God and we are to seek understanding of the scriptures. Not just Greek/Hebrew meanings but look in the overall context of the bible. The bible usually interprets itself.

What does the bible say about the character of God?
[Matthew 7:11]
If ye then, being evil, know how to give good gifts
unto your children, how much more shall your Father
which is in heaven give good things to them that ask
him?

[James 1:16-17]
Do not be deceived, my dear friends! Every good gift and every perfect present comes from heaven; it comes down from God.

So, if I look into the context of other scriptures, God will not send evil gifts but will only send good gifts. How do we then explain the evil spirit from the Lord?

Let's look at the context of the story of Job. Job, being a godly man, has yielded himself to God. God was in control of his life. When the devil wanted to attack Job, he had to go through God first and that he did. God permitted the devil to touch Job.

I believe that Saul was in a similar situation. God protected Saul and His Holy Spirit was upon Saul. When Saul disobeyed and chose to rebel, God was still in control and He allowed satan to take back control of Saul. It was like what the guest asked, God gave him up.

Whose fault was it? Was it God's fault when He gives us up to our desires. We have a free will and if we decide to live in bitterness, jealousy and rebellion, we actually give ourselves up again to evil spirits. However, when we are believers, we believe that our lives are still in God's control. He is sovereign and whatever happens to us is permitted by God. Do we then say that God sent evil into our lives? That all boils down again to the concept of free will. We can choose in life because we have free will, but we cannot choose the consequences. There is still cause and consequences in life. These laws are still in operation. It's only by the supernatural grace of God that sometimes we escape the consequences. Even David have to live with the consequences of his sin when he went with Bathsheeba. God may allow us to suffer the consequences, but His grace is always there and He is still in control. If we continue to yield ourselves to Him, all things can work together for good.

I believe that Saul also had that choice. He could have yielded to God even when he lost his "king"ship. But he chose not to and gave himself over to the evil spirit with the permission of God.

There's a very good article on this:
http://members.ozemail.com.au/~lkolberg/transcripts/C211C.html 

I don't have an answer, but I'll add some questions: 1 Samuel 18:10 “The next day an evil spirt from God came forcefully upon Saul. He was prophesying in his house, while David was playing the harp, as he usually did. Saul had a spear in his hand.” Or 2 Cor 12:7 To keep me from becoming conceited because of these surpassingly great revelations, there was given me a thorn in my flesh, A MESSENGER OF SATAN, TO TORMENT ME. 8 Three times I pleaded with the Lord to take it away from me. 9 But he said to me, "My grace is sufficient for you, for my power is made perfect in weakness." Therefore I will boast all the more gladly about my weaknesses, so that Christ's power may rest on me. And: 1 Corinthians 5:5 hand this man over to Satan, so that the sinful nature may be destroyed and his spirit saved on the day of the Lord. And: Rom 1:21 For although they knew God, they neither glorified him as God nor gave thanks to him, but their thinking became futile and their foolish hearts were darkened. 22 Although they claimed to be wise, they became fools 23 and exchanged the glory of the immortal God for images made to look like mortal man and birds and animals and reptiles. 24 Therefore GOD GAVE THEM OVER in the sinful desires of their hearts to sexual impurity for the degrading of their bodies with one another. 25 They exchanged the truth of God for a lie, and worshiped and served created things rather than the Creator -- who is forever praised. Amen. 26 Because of this, GOD GAVE THEM OVER to shameful lusts.
I'd like to understand this as well!