Question No. 56 asked of Dr. William Lane Craig on his web site sparked my interest in tackling freewill and evil.
“Why it is necessary that we be able to choose evil for us to have free will, while it is not necessary that God be able to choose evil for Him to have free will?” was the actual question.
For starters, I must admit my limits and refer my readers (all one of me) to Lewis or Augustine, Bonhoeffer or Athanasius for a detailed answer. Dr. Craig is certainly well thought out, but I always like to think things through myself.
Short answer: Good is defined by who God is, His very nature. Evil is turning from or disobeying God, and as we are like but separate entities, we have that choice. A choice that God does not have. God cannot turn from Himself, though He may do as He pleases.

Longer answer:
Let us begin by defining God’s qualities, as best we can quickly (listing them all is of course impossible) – Omnipresent, Omnipotent, Omniscient, (and Good) So, Infinite, Outside of all time and space, ie, not bound by them, and being God, that is, uncreated and the Source of all Creation, we can agree that He is the Ultimate Reality. He is so full of being that He gives it away. God simply Is. It is indeed His very name.
Now, is what God has Freewill, as we would describe it? No, as we humans would describe it, I think not.
If God is the Ultimate Reality and knows everything, then He certainly knows Himself. Complete self awareness is something that we cannot imagine, particularly in an infinite being. God is not in doubt as to what to do, He simply does. He is not at all in doubt when anything is put before Him for who is there to present something novel to the Creator? So, we come back to the fact that since He is the Source of all and is outside, apart from, all time and space, there is nothing that God could really choose to do that is outside of His own nature, which is itself, goodness. The source of goodness. It would mean there was something other than Himself, outside of Himself. This is not the place for an argument in Dualism, but I think its safe to say, God is alone (in a Trinity). (The true term, God actually has no plural.)
Now we must go about defining Good and Evil. Good is defined by who God is; all that God made was declared good. All of Creation is good. Christian theology tells us that God cannot make evil. Out of something Pure, only pure may come. Everything that we would call good usually means, uncorrupted, fresh, alive, beautiful. But because the created thing is not eternal, and infinite (1) it is therefore open to choice, this or that, being or reverting back to nothingness. What this means is that just as God Is, and good is, evil is not. Evil is not the opposite good, it is the lack of or the perversion of it.
The devil, even was once the most brilliant of angels, and now, as Lewis points out, is merely God’s ape. He can’t make anything himself, he can only twist what is God’s. Even the pleasures that you think sinful, are goods used in the wrong way or at the wrong time. Food is good, food taken in and of itself, apart from the intention God made of it, ie, as nourishment, is bad. Gluttony is nasty. Greed as well. Money isn’t evil, but loving it is. Sex is good, but separating it from the vows of marriage and bonds of the other loves due the marriage bed is a sin. Only God is infinite and good. Loving anything created (unless it is ordained, eg your neighbor – he/she is created in the image of God) is simply putting eternal hope on something that isn’t eternal. You are separating it from the fullness of its original intention. Note, even love, without justice, faith and honor leads to all sorts of envy, pride, greed and hate.
So now we have four things: God IS, therefore good IS, evil IS NOT, and God’s “freewill” is not quite the same as our freewill. God simply cannot violate His nature anymore than you can. Sinning doesn’t mean defying your nature, it means more of, defying the nature of God in you. God doesn’t have ‘the choice to do evil’ anymore than you have the choice to become a giraffe.
Lets sum up. Freewill does not mean just choosing between good and evil, as though there were only two choices, in this we have greatly limited ourselves. The truth of it is far more complex. You must choose this good or that good, this evil or that evil. With what we have discovered about good, there is an infinite number of “goods” available to us. All of creations sings it. I can choose a myriad of ways to help my neighbor, but far fewer ways to cause him ill. As a doctor in Iraq once said, ‘Violence (and evil) are for those who lack imagination…’ (2)
Thoughts?
(1) Christ is God and is eternal and infinite – See Lewis’ Trinity
(2) “Violence is for those who lack imagination. Does your country have no imagination?” Anonymous Iraqi doctor speaking on the war.