I read an account of a Christian on a college campus who was posed this question:
If evil and sin are the result of mankind’s free will, then why is it that people in heaven will have free will, but not sin? God made angels, and they do not sin. Why could it not be the case that God could have made humans that do not sin?
As the story goes, the Christian was struggling with the answer to the question.
I do not know the Christian named in this story, nor was I there at the time, so I am not posting their name here. However, it does not surprise me that many Christians would struggle with questions such as these, for Christians have too long not delved deeply into wrestling with such issues or seeking to give answers.
As to the last question, whether God could have made a world where people are free yet never choose evil, there is a way to answer this by investigating every logical possibility of mankind, freedom, and choice. I have given a lengthy response to that (see here). In short, the only way to achieve the greatest possible good is to allow people the freedom to choose whether or not to sin, and when given such a choice, sin will be chosen inevitably.
But regarding the first question, whether or not people in heaven will have free will yet choose not to sin, there is another aspect of this. Here I will give a long quote from Norman Geisler’s Systematic Theology:
[In heaven,] our freedom will be perfected. While all freedom involves self-determination, in order to test His creatures, God also gave them the freedom to do otherwise, that is the (libertarian) power of contrary choice. This freedom is still retained in fallen humans; however, it will not exist in heaven, where our freedom will be perfect and made more like God’s. Being absolutely perfect, God does not have the freedom to do evil (Heb. 6:18; James 1:13). Likewise, at the Beatific Vision, when we behold absolute Goodness, we too will no longer be able to sin.
Now, by God’s grace we are able not to sin (1 Cor. 10:13), but then we will be not able to sin. This is not the loss of true freedom but the actualization of it. Perfect freedom is not the freedom of being in bondage to sin; instead, it is the freedom of being delivered from sin. Again, heaven, like marriage, is not the deprivation of freedom but the fulfillment of it. We will one day be delivered from bondage, including bondage to Satan. (3.242-243)
Knowledge of God is knowledge of an infinite good; once one directly sees infinite good, it will no longer be possible for him to do evil, for to be directly informed in one’s mind by absolute good is to become completely conformed to it. Hence, the Beatific Vision makes sin impossible. Just as seeing absolute beauty will spoil one forever from longing for anything ugly, likewise, beholding the absolutely holy will overpower any attraction to or desire for the unholy.
Though heaven makes sin impossible, it does not destroy but instead fulfills our freedom. Heaven completes our freedom to completely love God, just as (analogously) marriage here on earth frees us to love the one to whom we belong. True freedom is not the freedom to do evil, but the freedom to do good. The essence of free will is self-determination, and if one’s self chooses to do only the good, then the fulfillment of it in a place where only good can be done is not the destruction of freedom, but the fulfillment of it.
God is both free and unable to sin; it will be likewise for us when we become most godlike, for the perfection of our freedom is the freedom from sinning, not the freedom of sinning. The best freedom is the freedom to do the best; beholding and loving the absolute best (which makes sin impossible) is the best thing we can ever do. (4.306-307)
Perhaps an analogy will help. A man is free to take drugs, but if he becomes an addict, he is enslaved to the drug. True freedom will then be the freedom not to take the drug. Likewise, we are slaves of sin (John 8:34) but true freedom will set us free from the bondage of sin (Romans 6:17).
If you would like an even more simple explanation, one rooted in Jonathan Edwards, think of this: When we are glorified in heaven, God will give us a new desire. We will not desire to sin, even though we are free.