The traditional Christian explanation for while evil and suffering exists in the world is the fall of Adam. Though certainly biblical, for non-believers, this view is unpersuasive. More to the point for this site, it isn’t what Christ taught.
Suffering Is Not Only Useful But Necessary
Christ taught that evil and suffering were part of God’s plan and make sense. Just as certain physical constants in the universe are necessary for life to exist, Christ teaches that evil and suffering are necessary conditions for a meaningful life (see this article on “satan” in the Gospels.) The fact that those who reject any meaning in life cannot understand this viewpoint doesn’t mean that evil is a “problem.” One doesn’t have to believe in original sin to appreciate Christ’s viewpoint.
We start by recognizing that the word that the Gospels translates as “evil” doesn’t mean that at all (longer article here about Good and Evil). The word usually translation as “evil” is poneros, which means burdened and base. It carries a sense of suffering as well as making second-rate choices.
For those who haven’t rejected the idea that life may have meaning, evil and suffering are necessary for the very simple, logical reasons that Christ talks about almost constantly.
While Christians teach this has to do with the fallen nature of man, Christ never says anything about man being fallen in any terms. He discusses what we choose to believe. When he discusses the purpose of good and evil, he does it in the context of the purpose of the world (mistranslated as “the end of the world“) and the purpose of adversity (mistranslated as “satan”).
There are two kinds of “evil” that people in the world: 1) evil caused by the actions by humans, and 2) suffering cause by nature (sickness, disaster, death).
Bad Acts by People and Free Will
The first category of evil exists as a necessary condition of free will. If evil actions by humans were not possible, we would have no meaningful choices. If no suffering (ours or that of others) resulted as consequence of our decisions, all choices would be equally good and therefore meaningless.
Those who think a loving God would have only made a world were human evil was impossible are simply saying that a loving God would be the supreme dictator: preventing us from hurting each other.
Or they are saying that God should have arranged the world so that all our decisions generated good consequences. However, logically, this means that we would have no decision to make. Every decision would be equally good.
Actually, Muslims believe that people are free to act, but every result is determined by God alone. In other words, they believe that our actions do not have consequences as such. It is always God that decide.
On the contrary, Christ attributes bad results to human choices. In his parable of the "seeds and the weed," the servants (us) of their master (God), asks in Mat 13:27 why "good seed" (a good source) has resulted in "weeds" (that which is false). The master (God) answers in Mat 13:28, that "hating humanity" or "a hateful person", obscured in translation as "an enemy" has done this."
When Christ cast out evil ("worthless") demons, he was able to free people of their bad habits and crazy impulse, but he still made it clear that they had choices. Even if we get rid of our demons, they will return unless we aim our minds toward another purpose (Mat 12:44). We let our bad impulses destroy our lives if we surrender to them, allowing the strength of our will to be bound by them. Our choice is between fighting against these forces and being bound by them (Mar 3:27).
Natural Disasters, Illness, and Death
So, why would a loving God allow natural disasters? Why are children killed in hurricanes if God is good? Doesn’t God love little children?
The answer has two parts. First, as a proof of a power greater than ourselves and second as a necessity for human achievement.
Christ makes it clear that not everything that happens in the result of our own actions. Human suffering from natural infirmities is not some divine justice. When people suffer, they are not being punished by God for their own actions. God may act for reasons that are beyond our understanding, but what we must understand from these events is that we don't control nature. God does.
However, in fighting against nature, we gain something valuable. Something we can only gain from the incentive of human suffering. If there were no suffering caused by nature, we would have no reason to care for one another. We would also have no incentive to learn more about the world and how it works. We love little children in the first place because they need us. If no harm could come to children, they wouldn't need us and we wouldn't have to love them so.
If the world was made up of candy canes and gum drops and every wish came true, why would we need each other? What could we do for each other?
From another angle, if we weren't ignorant and helpless in the face of nature, and if every need was gratified immediately, what incentive would we have to learn about the world? Why would we even seek any meaning in life beyond our pleasure?
Atheists see suffering as a proof that God does not exist. Their "logic" is that no good God would allow human suffering. By this thinking, any loving parent would simply give their children pleasure and nothing else if they could. But is this thinking true? Even for humans (disregarding the assumptions it makes about God)?
I offer a simple thought experiment. Imagine human science and engineering allowed us to attach our children to a pleasure machine. This machine would keep them alive, feed them, keep them comfortable, take away their wastes, and most importantly, keep them in a state of perpetual mental and emotional bliss through drugs and electrical stimulation. The machine would require nothing from them. They would not need to walk, talk, or even think. All they would know is waves of physical and emotional pleasure. They need know nothing of life and suffering.
How many loving parents would surrender their children to that machine? I suspect that very few would and those that did might be thought of as guilty of "child abuse" by the vast majority of humanity. Why?
People are simply rejecting the purpose of suffering without thinking about what it means. Christ suggests in Mar 3:26 that the purpose of adversity is to force us to rise up. Satan cannot throw out satan (Mar 3:23), or, more precisely, adversity cannot throw out adversity. We must do it for ourselves. We learn what works and what doesn't by acting against adversity.
Meaning and Purpose
Like children complaining that their parents won’t let them eat candy for every meal, we cannot complain that God doesn't give us what we want automatically. He will give us what we need, but first, we have to knock and lask to get the answers. As adults, w have to realize that this world has suffering, man-made and natural, as necessary conditions for our freedom and growth.
Life allows real suffering because suffering encourages growth. Real suffering is what makes real pleasure possible. Real pain means real joy. The world that Christ talks about allows real freedom and real consequences.
Life has a purpose and the purpose it growth, specifically, what people all through history have referred to as spiritual growth. Even pagans before Christ realized rising to it challenges is what gives life its meaning. Julius Caesar famously wrote, "I love the name of honor more than I fear death." What is a passion for honor other than a spiritual meaning to life?