Because God is good, because He is infinite, because He is perfect, because He is ultimate and no standard exists above Him, evil is that which displeases Him. Evil is not something that exists like a goop creeping through the seams of reality. It is not a Force that can make you wear a black cape, a helmet, and give you asthma. Evil is an adjective. It is an adjective used to describe those actions of man (and their effects) that are contrary to the nature of God.
Since (in the Christian system) God is the standard of good and evil, the question can be reframed. How can an all-good, all-powerful God allow things in His creation that displease Him? .. Evil exists in the world because God created man as a free agent. With freedom (obviously) came the ability to do both right and wrong, to please and displease God. But what a price. Why is human freedom so important that billions of lives had to be sacrificed on its altar? Because a Heaven populated by free souls is a better world than a Heaven populated by puppets.
What is the world? What is it for?
It is art. It is the best of all possible art, a finite picture of the infinite. .. This painting is by an infinite Artist. It is a reflection of Himself (could there be a better subject?), worked out in colours, lives, and constellations, in a universe that to us seems endless but is to Him a mere frame, a small space, a confining challenge for His artistry.
If we live in art, struggling in the boundary between the shadow and the light, unable to see the whole, how can we begin to judge? How can we presume to talk about a better painting, a better novel, when we see only a single line, a single page, and it brings us grief?
Any single needle can complain. There is death in those branches. Surely I could be full and green, surely I need not be in the wind, connected to the struggle? There is a shadow sprawling across me. I am cold. Can we bring in more light? The contrast could be softer.
And so we all speak. Each of us wanting our own position a little more comfortable.