The next paper I have to write for my Modern Philosophy class is about the Problem of Evil, which is a logical problem that's created when ones considers the power and goodness of God alongside the darkness and evil that exist in the world. Theoretically, if God was all-powerful, He could rid the world of evil, and if He was all-good, He would do it. I don't like absolutes, but I still believe that God is at least powerful enough to eliminate most, if not all, evil from the world and good enough to want to, so why doesn't He?
There are many reasons.
Moral Evil, which is the evil things people do and the consequences that result from them, is by definition the results of human action. God's not responsible for those acts; we are. Arguably, God shares some of the responsibility, just as any parent who gives a child a potentially dangerous tool can be considered somewhat responsible if the child uses that tool to hurt themselves or someone else. Still, this suffering can be beneficial, if we learn from it. We can learn from our mistakes and the mistakes of others, and suffering for any reason can stretch one's soul.
Natural Evil, which is suffering and imperfections that naturally occur in the world, is more directly linked to God, but are no less instructive. While it takes a bit more abstract thinking to learn moral lessons from the behavior of nature, the suffering that results from that behavior can stretch one's soul just as well as suffering caused by moral evil.
While we may think that the world might be better without any moral or natural evil in it, it's important to remember that those evils and the suffering they cause play an important role in God's plan. From evil and the suffering that results from it, we can learn essential lessons that we could hardly have learned otherwise. It could be true that evil itself is actually essential to God's plan, which might explain God's creation of Satan, though I'm less sure about that. What I am sure about is that there are good reasons why evil exists in this world and why God is wise enough not to eliminate it.