why evil exists

This is the best - and most logical - explanation I've read of "the problem of evil". It begins:

The first step in answering the problem of evil is this: We've got to get clear on what this thing "evil" actually is. It does seem to follow that if God created all things, and evil is a thing, then God created evil. This is a valid syllogism. If the premises are true, then the conclusion would be true as well.

The problem with that line of reasoning is that the second premise is not true. Evil is not a thing... evil has no ontological status in itself.

The word ontology deals with the nature of existence. When I say that evil has no ontological status, I mean that evil, as a thing in itself, does not exist.

Let me give you an illustration to make this more clear. We talk about things being cold or warm. But coldness is not a thing that exists in itself; it has no ontological status. Coldness is the absence of heat. When we remove heat energy from a system, we say it gets colder.

"Cold" isn't a thing. It's a way of describing the reduction of molecular activity resulting in the sensation of heat. So the more heat we pull out of a system, the colder it gets. Cold itself isn't being "created." Cold is a description of a circumstance in which heat is missing. Heat is energy which can be measured. When you remove heat, the temperature goes down. We call that condition "cold," but there is no cold "stuff" that causes that condition.

Here's another way of looking at it. Did you ever eat a donut hole? I don't mean those little round sugar-coated lumps you buy at the donut shop. I mean the hole itself. Donut holes are actually what's left when the middle is cut out of a donut. There's a space called a hole, a "nothing," the condition that exists when something is taken away. Same thing with a shadow. Shadows don't exist as things in themselves; they're just the absence of light.

Evil is like that. Evil isn't like some black, gooey stuff floating around the universe that gloms onto people and causes them to do awful things. Evil is the absence of good, a privation of good, not a thing in itself.

When God created the universe, he created everything good. He made a universe that was perfectly good. Everything was as it should be. After God was completely done with creating everything, something happened that reduced the good in the world. That loss of good is called evil.

That's why in Genesis 1 we read "it was good" many times. From the record we know that God didn't create evil. But something did happen in which evil - the loss of good - took place, and as a result a lot of other grotesque things came about.

So donut holes don't exist; they're just the absence of donut. Shadows don't exist; they're just the absence of light. And evil doesn't exist; it's just the absence of good.

and continues:

Now the real question at this point is, "Was it worth it? Good can come out of evil, but was it worth it in the long run, the measure of good that comes out of the measure of evil in the world?" And my response is that the only One who could ever know that is God. You and I couldn't know that because our perspective is too limited. Only God is in a position to accurately answer that question.

Apparently God thinks that, on balance, the good is going to outweigh the evil that caused the good, or else He wouldn't have allowed it to happen. Christ paid a tremendous price, an example of the tremendous love God had for us. God would not be able to show His sacrificial love unless there was something to sacrifice for.

Here's the problem, and this is why we don't think that, on balance, it's really a fair trade. We think that life is about giving us pleasure and making us happy... God's purpose for creating us was to develop us into certain types of people who were fit to spend eternity with Him. He does that by conforming us to His image by helping us grow through the process of living in a fallen world...

In God's mind, the goal of the process - being conformed to the image of His Son - is a much greater good than the bad of the evil that we have to put up with on this earth. The balance is definitely on the side of good.

I would expand on that argument by pointing out that this greater good of living in eternity with Him would by definition infinitely outweigh our temporary temporal suffering.