Before we can begin to discuss the whole why does God allow evil debate, we need to have an understanding of what evil is.
While evil is real, it is not a thing. The argument goes like this:
1. God created all things
2. Evil is a thing
3. Therefore, God created evil
Then we get all upset because we want to know how to square the idea of a good God creating evil. The error is found in the second premise, that evil is a thing. Evil is not a thing, even though it is real. Evil does not exist on it's own. It only exists as a lack or deficiency in a good thing.
Evil is like the rust in a car: If you take all of the rust out of the car, you have a better car; if you take the car out of the rust, you have nothing. Evil is like a cut in your finger: If you take the cut out of your finger, you have a better finger; if you take the finger out of your cut, you have nothing. In other words, evil only makes sense against the backdrop of good. That's why we often describe evil as negotiations of good things. We say someone is immoral; unjust; unfair; dishonest, etc.
So evil can't exist unless good exists. But good can't exist unless God exists. In other words, there can be no objective evil unless there is objective good, and there can be no objective good unless God exists. If evil is real - and we all know it is - Then God exists. Therefore the question of evil is better used to prove God's existence rather than disproving His existence.
 - Stealing from God by Frank Turek