Someone asked me, "If God knows someone will become evil, such as Hitler, then why would He allow them to be created?" I went to Abraham offering up Isaac as a sacrifice and how God said, "for now I know..." Maybe God chooses not to know if we will become evil since we are given free will. Is there a better answer?
The questioner assumes that God knows all our choices at the point of birth. If that were true, then how can God call Israel's children innocent? "Moreover your little ones and your children, who you say will be victims, who today have no knowledge of good and evil, they shall go in there; to them I will give it, and they shall possess it" (Deuteronomy 1:39). If it were possible to know at conception who will be evil, then Solomon's statement would not be correct: "Truly, this only I have found: that God made man upright, but they have sought out many schemes" (Ecclesiastes 7:29).
The truth is that until people make choices, you don't know which way they will choose. That is why God said of Abraham, "Do not lay your hand on the lad, or do anything to him; for now I know that you fear God, since you have not withheld your son, your only son, from Me" (Genesis 22:12). Once a person begins making choices, it is possible to determine what further choices he will make in many things. But some choices require waiting and seeing what a person will do. I don't think it is God choosing not to know. I believe that somethings are just not knowable in advance.
But that just delays the question a bit. Once it is clear that a person is not ever going to do right, why does God allow that person to continue to live? Again, a part may be that God is demonstrating patience to allow a person to change (II Peter 3:9). Where we might give up and think a person will never change, it is possible that God sees that a possibility exists that we ignored and allows the person more time to make a choice.
Yet, even the wicked have a place in God's plans. Paul tells us there is a reason why the tyrant Pharaoh rose to power in Egypt. "For the Scripture says to Pharaoh, 'For this very purpose I have raised you up, that I may show My power in you, and that My name may be declared in all the earth'" (Romans 9:17). The wicked nations were used to punish Israel when they strayed from God. Even today, they may be used to chastise the righteous to give us reasons to grow (Hebrews 12:5-13).