What is Evil?

Artistic Depiction of Satan

If God exists, why is there evil? That's a common question asked by atheists and Christians alike. Whether you believe in God or not, it's hard to understand what evil is. That's because of the way our society deals with evil. As a society, we treat evil like a condition that people have, and like something that had to be created. But that's not what evil is. Evil is a corruption of good. Let's elaborate on that further.
When we discuss what evil is, we discuss something we perceive as negative. The human body is great, but it experiences sickness. Does that mean a sick body is bad? No. A sick body is merely a body corrupted by a virus or bacteria. Another analogy for evil is cold. Cold isn't something that actually exists. It's a lack of heat. To apply the analogy, good is like heat. Good is God. Anything that isn't close to God is therefore evil. That's where the corruption comes from.
The next question is why that corruption exists in the first place. If God created everything to be good, how did it get corrupted? This is a little long-winded, so get comfortable.
First, we start at where evil came first. God did not create evil. Rather, he created angels. At the time of their creation, he revealed parts of his plan to each one of them. Remember, God and the angels are metaphysical, so they aren't bound by time. At their creation, with the knowledge of God's plan, some of them "fell." The Fall refers to those who chose to take their own power. We can agree that this seek for power on their part is bad. So since we can agree that they did something evil, how did it come about? If everything was good, what made something have a bad thought? Well, the thought of being second to God served as a temptation for some. In the beginning, authority and power were good. They were corrupted during the Fall when some demons chose to take power and authority for themselves  Their autonomy allowed them to take what was good and turn it against God. If you have a screwdriver that you use to build, you can just spin it the other way to make it destroy. In the same way, power is a neutral object. It exists, but can be good or bad. In the Fall, some angles turned the screwdriver the wrong way. They simply turned it in a different direction and started the corruption.
Here's another analogy. Think again of the human body. It's pretty good. There are some sicknesses that are corruptions, however. So separate from these corruptions, lets see if the human body is perfect. It isn't. It has some corruptions that exist for no reason. People have wrong numbers of chromosomes, different DNA structures that become cancerous, and corruptions in the body that exist without outside influence. These exist because corruption is not an evil in itself. I've been wrong in this article calling it "corruption." It's actually change. You can change something for the better or for worse. Change itself, the action of change, is not swayed either way. Evil stems from change. People change and use free will to turn away from God. That caused the Fall, and it causes sin to this day.

Now, this concept does seem to fall to an infinite regression. While it would seem to suggest that evil is a corruption, that would also beg the question of what corrupted force corrupted it in the first place. If you thought that, I commend you, you seem intelligent. The answer to that is a clarification mod a corruption of good. Since God is infinitely good, anything away from god is the perception of evil. That means anything finite is subject to evil. Since God is also infinite, anything he creates is finite. Creating something infinite would clash with God's infinity itself, meaning he would never do it. Thus, everything God creates is finite and thus subject to error. You can't have two infinities. It's just not possible under the whole concept of infinity.