Don't say I didn't warn you.
The first thing we need to establish is whether God has free will. There are actually arguments to say that He does not. There are two main of which I am aware.
First, God is always good, and therefore He must always do the good thing. He has no option or freedom to do otherwise. The refutation of this argument is that it elevates goodness to some level independent and superior to God. If that is the case, then God isn't God, goodness is now God and God is just a servant of goodness. If we're talking about the first cause, omni- God, then the relationship is backward. God is good because He is good, not due to some independent criteria for goodness. Whatever God wills is good, and God has the freedom to will as He chooses.
Second the foreknowledge. The same argument of predestination and free will can apply to God as well. The problem is as follows. God knows the future. The future exists because of God's will. So since God can't be wrong in His future knowledge He can't Will other than He will Will. Or less confusingly, He can't do other than He will do. Duns Scotus spent a lot of ink refuting this. The basis of his argument is that God knows the future only because God Wills the future. If He willed otherwise, the future would be otherwise and He would know otherwise. At the time of His act of willing, He is entirely free and His foreknowledge is a consequence of His will and not the other way around. This gives the basic idea that God knows because God wills, and I will come back to this eventually.
So in this situation, God willed Saul to be King. He could have willed otherwise if He wished, so in that context the idea of regret is not absurd.
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