One of the deepest, most difficult issues for any theology is the problem of evil. It is possibly the one question that separates each religion from the others, since all religions' definitions of God or the divine and their worldviews are built around explanations of the good and bad in the universe.
Actually, it is two questions. One is why do bad things happen to good people, and the other is why people are sometimes bad. They really are two entirely different issues, but if you are talking about evil as a theological concept you can't separate them completely. Theologically whether human beings are themselves evil, or whether there are negative things happening in the universe which affect people is to some degree the same question. It has been dealt with in two primary ways, either by accepting the idea of dualism, or the war between forces of good and evil, or by trying to reconcile them in monistic worldviews.
There was a clear dualism in the ancient religions of the Middle East from which our Judeo-Christian heritage came. There was a good creator, and there was the Enemy. When there were a lot of different gods, that was easy enough to deal with, but when there became only one supreme god who created everything that there was, the problem of evil began. Any kind of monistic worldview has to deal with the issue of the counterforces of good and evil, and so far I don't believe that anyone has been able to discover a satisfactory resolution.
Zoroastrianism, which bred the Mithraism that almost overcame infant Christianity and still has its adherents in the Parsees of India chose dualism pure and simple, and they were the only world religion to do so. They said that there were two forces, one for good and one for evil, always in battle against one another. The human purpose is to enlist on the side of the force for good which eventually will win. The good force was the creator, but it was not alone in the universe but co-eternal with the destroyer. Human beings were created to fight on the side of the good, but they were given free will, and could and sometimes would choose to battle for the other side, the side of evil.
However, mystics in all religions, even Zarathustra who was the founder of Zoroastrianism perceived a basic unity in our universe. We are all one, and we are not separate from God who is one with us and with the universe. Though mystics in all religions have said precisely that, it does not answer the basic questions of meaning, and in fact poses new ones. Theologians are always having to try to clean up the loose ends that the mystics and prophets leave for them.
The theological problem of evil is the hardest one to manage if you are dealing with the monistic ideas of mystical vision, which you will find to be the preference of thinkers in all religions, even those whose popular version is polytheistic like that of Hinduism, or indeed even with the monotheistic teaching of western religion. That includes both branches of the problem.
There are underlying dualistic tendencies in most popular forms of religion. Flip Wilson got a lot of sympathy with his tag-line, "The devil made me do it." Usually for most people that is sufficient. Although they will argue that God is the creator, they have no problem accepting the devil as the official enemy. He simply rebelled against God because God believes in the freedom to rebel, or as Milton argued in Paradise Lost, merely from pride. (Wherever that pride came from.) That's why human beings are bad sometimes, too. They have been given the freedom to rebel, and so they do so. There is no explanation of where the evil in them came from, or the desire to rebel. It is just assumed that that is part of human nature without trying to decide where human nature came from.
However, that is popular religion in the west, which has insisted so strongly on monotheism. In the east they just have a lot of good and bad gods and pretty much let it go at that. Deeper thinkers in both parts of the world, however, have to do a lot of figuring out of how things ought to work without either lessening the all-encompassing nature of ultimate reality or making it less than perfect.
Generally we have three primary ways of dealing with the concept of evil, either as human sin or suffering, or the clear difficulty other animals and even plants often have in achieving minimum survival -- much less having to worry about the quality of life as we do. We either sentimentalize it, romanticize it, or deny it. It is very hard to accept it as something that exists as an active force inimical to the good we cherish if we prefer to believe that the universe is basically a friendly place and is all of a piece, as most of us do prefer.
Denial of evil is the most intellectually acceptable stance. The most consistent philosopher cum theologian I have ever read was Spinoza. Generally in reading philosophers from Plato to Whitehead you can find at least one spot where an argument -- and nearly always it is a pivotal one -- doesn't follow directly from a previous one. Steps are skipped which are necessary to whatever their argument may be. That is not true of Spinoza. His work is spare but extremely dense and each proposition follows from the one preceding it with absolute inevitability. He argues that the universe is one and is good. Those are axioms which he assumes because it exists as an organic whole, and what happens in it must be for its benefit. Whatever is for the benefit of the universe is by definition good. That means that sin, suffering and death are good, because ultimately in some way that we may not be able to fathom with finite minds attempting to understand the infinite, they are for the benefit of the universe.
Without being so logical, more traditional theists say almost the same thing, but they ascribe it to God and know it by revelation rather than logic, thereby managing to personalize it and say that what we conceive of as evil is actually good for us personally. God is good; God is infinite; all that happens is according to the will of God; therefore it is good, and if we don't think so it is simply because our finite minds cannot understand the infinite, beneficent will of God.
The eastern religions influenced by Taoism seem at first glance to be able to deal with the paradox of one reality which contains both good and evil. Basically with the yin and yang they say that one is defined by the other as everything is defined by its opposite. Without evil good could not exist, just as evil could not exist without good. Neither is more or less acceptable than the other because they are necessary to each other. I think, however, that this, too, denies the reality and the horror of the kinds of suffering that exist for some people and indeed some things, because it excuses it. To excuse it is to deny its significance and a basis for the righteous anger with which I think it should be met.
Hinduism and Buddhism explain suffering as caused by clinging to the reality of an unreal world. Since all that we see about us is maya, illusion, then so is suffering illusion, and it is caused by the acceptance of that illusion as being real.
Denial of evil is the stance with the best credentials, but it is not the only way we deal with it both in our culture and even within our Unitarian Universalist tradition. One of the most emotionally satisfying ways is to sentimentalize it. The poor boy who killed several people in horrific ways is far more to be pitied than censured because he had a deprived childhood, so what he did wasn't all that bad. The child who is born with Down's syndrome was put here to make all the people who come in touch with her better people -- which may indeed happen, but doesn't make her life any longer or richer.
Or we romanticize it. The devil is a gentleman. One of the people who did that best was Milton in Paradise Lost. In fact, in that work, the devil is actually the hero. In literature we often have exciting villains who are far more interesting than their virtuous opposers. People in prisons who manage to make themselves known receive many proposals of marriage. One of the reasons I was impressed with C. S. Lewis's way of dealing with the issue in the reading that I gave you from Perelandra was that he made it perfectly clear that pure evil is purely horrible, that Hitler and Charles Manson, Jeffrey Dahmer and our latest mass murderers are not to be excused. Neither, it seems to me, can God or the universe, or any one or thing be excused when there is unmerited suffering and destruction. That does not mean, necessarily, that they are to be blamed, either.
There is one way of looking at the problem, either from a theistic or non-theistic point of view, either in the west or in the east, which has the force of logic behind it. The theistic idea says that God, the creator, created good and evil, both of which are real. It is not up to us to make value judgements of what God has made, but merely to worship the power and the glory that is God. That's fine if you are into the worship of power, I suppose.
Non-theistically -- and this is an argument to which I partly subscribe even with enthusiasm -- the universe is random. Things happen as they happen, and we need neither praise nor blame. Justice and mercy are human concepts which cannot be attributed to the universe which has no will either to good or evil, but merely contains everything within it. Thus the efforts of most religions to even things out, either in an afterlife or successive lives, are not a part of ultimate reality. There are not either, of course, any injustice or cruelty as part of the overall plan of things. They are simply irrelevant concepts in relation to the universe, or perhaps even to God. Though that is very convincing to me when I see suffering which does not redeem the sufferer, when I see pain that is destructive rather than producing growth, when I see neglect and ignorance and unutterable selfishness unlightened by any sense of wrong, there seems something missing from this formulation.
I think I agree with G. K. Chesterton when he said that he couldn't understand what people meant by the problem of evil. He expected evil. What he wondered about was the source, the basis, of so much good in the world. The universe, if it is indeed random, will contain both good and evil. However, there seems to me to be, as to Chesterton, something creative, something loving, something good, some process which is not random which pushes or pulls or moves somehow all living things and perhaps the universe itself to greater fulfillment and greater good. Even in the face of dreadful intentional evil such as that perpetrated by the Nazi war criminals, though the evil itself was real, and the people I think should be called evil, they had the conviction that they were not evil at all, that what they were doing was right and proper. Though some behavior is unforgivable and the people who commit it are entirely culpable, the intention of good is still there, and so is the effort toward it in their self-justification.
So I fall back into dualism of a sort. I do not see the random universe as co-extensive with meaning. It is not the sort of dualism which says that there is a force for good and also one for evil, nor do I deny the reality of evil. It is real as good is real and as the universe is real, and the universe is indeed one. We are made of the same substance as the stars and are sisters and brothers of the smallest atom that exists. Indeed all of it is part of the rest of it. Nevertheless, I believe that there is beyond that, not force nor being but something more which is worthy of worship.
Worship itself means concern with that which is of worth. To me that does not mean a God of power nor a universe in which I am contained and indeed perhaps contain as the mystics say, nor does it mean to deny or distort the evil which is real. It means that I do worship the good, and I perceive it as beyond the finite goods within ultimate reality, a process toward the good in which somehow we all have our part.
Unitarian Universalists since their early history in which they denied that God was three persons have a stronger tendency than most to a strict monistic point of view about ultimate reality. Not only do they wish to say that God is one, but that everything is one. Our ultimate concern is the same as ultimate reality. I'd like to do that myself. In spite of this tendency our early history was much more concerned with the goodness of humankind than the one or threeness of God, and much as I would like the simplicity of the monist worldview, I am much more concerned with goodness, fulfillment and truth than with a view of the universe which subsumes good and evil into itself. This must deny the reality of evil or its significance and therefore its clear-sighted rejection. We need not consider a problem of evil; rather we need only commit ourselves to the single-minded choice of the good which calls us.