A curious verse

¹⁴ Now the Spirit of the Lord had departed from Saul, and an evil^[a] spirit from the Lord tormented him.

from NIV


¹⁵ And Saul’s servants said unto him, “Behold now, an evil spirit from God troubleth thee.

from KJV

 
¹⁵ Saul’s servants said to him, Behold, an evil spirit from God torments you.

 from AMP


The question is that why God sent an evil spirit to torment Saul? we know that All things work for the good of God. God is always in control. He is in control, has control over everything in this universe. He has control over demons, angels, everything. So evil spirits are under his control, though it was not created by Him. God in this case simply allowed the evil spirit that was wanting to torment Saul to do what it desires. God is always sovereign, regardless of the circumstance. The reason why God allowed it to happen comes from a variety of reasons, but whatever the reason, 

28 And we know that for those who love God all things work together hfor good,8 for ithose who are called according to his purpose.

Everything work together for good!

so, just remember

 
Trust in the Lord with all your heart
    and lean not on your own understanding;
⁶ in all your ways submit to him,
    and he will make your paths straight.<sup>[a]

Amen!</sup>