"Even though I walk through the valley of the shadow of death, I will fear no evil, for You are with me; Your rod and Your staff they comfort me" (Psalm 23:4).
At one time or another every rational person will ask why evil exists. Unfortunately, most who ask that question will not take to the time to consider what evil actually is. Without understanding the nature of evil, one cannot hope to understand the evil itself.
Evil is sometimes defined as the absence of good. This definition needs further clarification, of course, but the key point that needs to be grasped is that evil is not a physical or supernatural thing -- it is an absence of something in a relationship -- and is not a something in and of itself.
Moral evil is one of two broad categories used to describe evil; the other category being natural evil. Let's will deal with them separately.
Moral evil results from the exercising of our free will. If we freely choose not to do that which is good, evil results. For example, a person covets someone's possessions, acts upon the desire and steals the possessions, and thus harms the person he stole from. Had the potential thief not acted upon his covetous desires -- which would have been the good thing to do -- no harm (evil) would have occurred.
Natural evil is generally viewed as that which occurs as result of natural events (for instance, deaths or injuries due to tornadoes and earthquakes). Tornadoes and earthquakes are not in themselves evil. They serve a vital function in the natural order, but when a person and a tornado share an identical location harm will result. The question really is not why God allows natural disasters to harm people, but rather, why is there death and suffering -- any kind of death and suffering -- in the first place?
We live in a cursed creation (Genesis 3:17-19). Physical and spiritual death are aspects of this curse. When viewed in light of a cursed creation, death is not evil. Death spares the righteous person from experiencing no more evil than is necessary: "The righteous perish, and no one ponders it in his heart; devout men are taken away, and no one understands that the righteous are take away to be spared from evil. Those who walk uprightly enter into peace; they find rest as they lie in death" (Isaiah 57:1-2). Second, death helps to restrain evil, and thereby protects society from becoming completely reprobate. This was evident in Noah's Flood -- humanity had become almost entirely reprobate, and only righteous Noah and his family were spared: "So God said to Noah, 'I am going to put an end to all people, for the earth is filled with violence because of them. I am surely going to destroy both them and the earth'" (Genesis 6:13). Physical death is not the end of personal existence -- it is an event that marks a transition. As long as salvation history is being played out in a cursed creation, which God has ordained that it must be, death -- which releases us from the cursed natural order -- can be considered a gift of God.
Few people enjoy suffering. But suffering produces positive characteristics that, evidently, cannot be acquired in any other way: "We also rejoice in our sufferings, because we know that suffering produces perseverance; perseverance, character; and character, hope. And hope does not disappoint us, because God has poured out his love into our hearts by the Holy Sprit, whom he has given us" (Romans 5:3-5). Suffering is an aspect of God's loving discipline; we learn through suffering: "Endure hardship as discipline ... No discipline seems pleasant at the time, but painful. Later on, however, it produces a harvest of righteousness and peace for those who have been trained by it" (Hebrews 12:7, 11). Finally, suffering keeps us from thinking more highly of ourselves than we should -- it is a reminder of our mortality, and of God's sovereignty: "To keep me from becoming conceited because of these surpassingly great revelations, there was given me a thorn in my flesh ... Three times I pleaded with the Lord to take it away from me. But He said to me, 'My graces is sufficient for you, for My power is made perfect in weakness'" (2 Corinthians 12:7-9).
Though we, as long as we are on earth, will likely question the evil that we experience, we should take comfort in the knowledge that even our suffering has been ordained by God for our benefit. Someday we will fully understand the reasons for undergoing these trials: "Now we see but a poor reflection as in a mirror; then we shall see face to face. Now I know in part; then I shall know fully, even as I am fully known" (1 Corinthians 13:12). We can also take comfort in knowing that our Creator, through Jesus Christ, experienced suffering and death; He shared in our suffering so that we could be saved.
"Surely He took up or infirmities and carried our sorrows, yet we considered Him stricken by God, smitten by Him, and afflicted. But He was pierced for our transgressions, He was crushed for out iniquities; the punishment that brought us peace was upon Him, and by His wounds we are healed" (Isaiah 53:4-5).