A question that repeatedly arises in my line of work is, "Why does God permit people to go to hell?" Sometimes, religious people answer that this is because we are free, and so God allows hell in light of freedom. But doesn't this damage God's sovereignty over human wills? Isn't it cruel to create creatures in the first place, if He knows and permits them to suffer eternally? Why doesn't He either not create them or just annihilate them after death? Here's a short answer to the question, which I hope helps:
First, the problem of evil, it seems to me, is somewhat simply solved. We know the answer with certainty, even. So: if evil exists (which we perceive) and God exists (which we know from natural reason) and God is omnibenevolent (ditto), then there is a clear logical "out": the only way evil could exist is if God permits evil in order to achieve or permit a greater good. But this technical solution, while logically satisfactory, leaves us with a lot more questions. The problem lies in imagining what that "greater good" is, which is what leads us to feel the answer is non-satisfactory. We want to know what it is that could lead God to allow so many bad things to happen in our world. This is where I think we misunderstand Christianity if we forget that it is a direct answer to the problem of evil. I'll get back to this below.
Second, the most disturbing kind of evil God allows seem to many people to be the evil of eternal torment and moral evil. But many people have pointed out that happiness has to be free. Happiness is connected with what it means to be a good human being. Insofar as human beings are rational and free, achieving happiness should involve freedom. Consequently, there is a definitionally important part of even heavenly happiness which entails that we have to freely choose it for it to be human happiness. Happiness cannot be forced.
Third, it is true that God has sovereignty even over the will. His grace causes each person to convert, and does so efficaciously; without overriding our free will, He brings about the opportunity and power to convert, and He causes us to actually turn to Him (otherwise we couldn't choose Him in the first place, living in sin). Because of this, we can say that God could theoretically create a whole race of free people who all voluntarily convert - a whole race of people like the Blessed Virgin, for example, predestined to glory. So the freedom of human beings entails only the logical possibility of sin, not its actuality.
Nevertheless, even here, precisely because of this, the prevention of actual sin in this way is entirely a gift - it is not something the human being deserves, as it goes above and beyond their natural powers (which ordinarily would lead to falling away into sin). So it would not be unjust even if God did not actually bring any people to conversion; rather, it would only be unjust if God did not give everyone the real ability to convert. But, as we know by faith, God gives that ability to each person by "sufficient" grace; He gives everyone an opportunity, an inspiration, and real power to perceive and choose salvation. Everyone gets it, nobody is left out. What seems a problem, to the person who objects to Hell, is that they don't all actually convert.
Fourth, however, even in grace, God foresees each person as a person, not as a tool in His designs. He wills all of their acts in the future as a whole, despite having the power to intervene at each point in their lives; a person is not merely an atomic series of acts but a subject who has a relationship with Him. There is thus no reason for God to choose to create this person rather than that one; He isn't creating them based on a particular act that they do or not, but on their free personality as a moral whole. Each person is valuable as unrepeatable, with their own history, feelings, and choices. While one person might sin and another not, and God's grace might preserve one person and not another, this is what happens when God foresees and creates them as persons in eternity. Even when He chooses to create Mary over Sue, He did so not because He wanted a certain series of events, but because He willed them as persons to exist. He loves them entirely and holds all of their free acts together in His love that caused them to exist. So, in particular, there is no "reason" to create a person who goes to heaven rather than hell; God is willing each as a free acting person who He will not override in their free decisions. If they choose Hell, it is entirely their own decision and God loves them to the extent that He allows their free choice even against Himself. That's why both Hell and predestination together is a mystery - it is an act of love, pure and simple, just like the decision to create the universe rather than not. It's mysterious, but it makes a whole lot more sense when we see it's a very similar kind of thing to loving our neighbor, not for anything they did, but just because they are other people.
Fifth, this leads us to the "good" of allowing hell. While particular persons can't be created for a specific reason (in the sense that each is infinitely valuable, even if God foresees and permits them choosing to go to Hell). He permits it because of the real choice people had to sin. The fact that some actually fall away makes clear that each person had the ability to follow freely, and that nobody was forced to accept salvation - it was a gift to be accepted or rejected. He does not want people to go to Hell and gives them every opportunity to avoid it, but He respects their freedom despite all that. He creates them merely knowing that they will. In C.S. Lewis' The Great Divorce, there is the image of God descending continually into Hell and offering to bring them out - His mercy extends even to those in Hell and it never ends, even if they never accept it. We might think, for example, of how a lover, to be truly caring for the other person, lets them leave him and go after someone else. They respect even the choice to not love them in return precisely as an effect of how much they love their beloved. So, even though not wanting it to happen, they let it happen because they see that person as infinitely valuable.
This is why the Cross is the best answer to Hell. We see how much Christ forgave His persecutors and loved them, even after they sinned against Him. The descent into hell of Christ after His death witnesses that even those in hell had a chance to hear Christ and follow Him to paradise. If they stay there, it is because they want to - they are trying in eternal despair to find delight in something that is not God. In that sense, everyone in hell is there precisely because they want to be and don't want to be forced into heaven. God's mercy is present even in Hell. God's decision not to annihilate them is for the simple reason that He loves them and wants them to be able to do what they want, even if it leads to their unhappiness eternally. In that sense, despite all the pain and suffering, the damned don't want not to exist, and to cause them to do so would be to take away the only good they actually have - their own wills and personhood. It is also important to see that this mercy is unfathomable. We don't know how many people are in Hell or will be at the end of time. Hell could even be practically empty, for all we know. God keeps it a mystery, held in His love and mercy which extends to us. What is important is that His love desires us to avoid it, and that we ourselves are the only obstacle to that love.
Of course much more could be said about this problem and I don't plan to answer every objection. But the key is to understand that God views us as persons, not as robots or as a series of acts. The Cross is part of the answer because we see there that God is a person, live ourselves (and three Persons, ultimately), who loves each of us intimately and individually. He took on suffering and death, even descending to Hell, so that no person would be left untouched by His mercy. Christ did not see us as tools or instruments in His plan, but as infinitely lovable. We are each the lost sheep, the wounded Israelite on the road to Jericho, and the missing coin. This is both why He rejoices over our conversion and allows us to hurt Him. But, in the end, He is praying all the while that we be forgiven. It is only my will that stands between myself and God's desire to bring me into His love.