Defenders of the Christian faith settled on a definition of evil many centuries ago. Evil is a lack, a privation, or a corruption of what is good. It is not the absence of good because evil does not exist in and of itself. Evil is like a wound in an arm, rot in a tree, or rust on a car. It transforms what is good into what it ought not to be. Evil, therefore, is not the absence of good.
There are two kinds of evil that trouble mankind: moral evil and natural evil. Moral evil occurs when we choose to do evil. 9/11 is a moral evil. The massacres in San Bernardino and Orlando are a moral evil. Natural evil is a corruption or privation beyond human choice. Ebola is a natural evil. Human disabilities are a natural evil. Sometimes these two evils intertwine. Take, for example, an area plagued by malnutrition that was caused by deforestation carried out by a company greedy for resources. Here is a reprehensible, immoral act with natural evil as the consequence.
Integral to the Christian view of evil is human free will. Self-determinism, in turn, ultimately rests on our power to choose apart from any external causation. What I choose to do or think is up to me and no one else. The capacity to freely choose means I also have the capacity to love. The only meaningful love is that which is freely given. Is it possible to love out of compulsion? If so, is that truly love? The greatest of human virtues is meaningful if, and only if, we can freely choose.
The same free will that leads to the highest human potentiality also unlocks our darkest capabilities. We cannot learn to love without free will. And yet, to equip humanity with free will was a risky proposition. The power to choose to love exposes us to the temptation to choose otherwise. Why does evil exist? Because we were given the capacity to love.