Why does evil exist?

I personally think that the problem of evil is usually the existence of how the existence of God can be reconciled with the existence of evil in the world. The problem simply stems from basic beliefs or assumptions pertaining to the attributes of God. God is perfectly good, omniscient, and omnipotent. From this, such a God should want to prevent evil, yet much evil exists. There have been many proposed solutions to the problem of evil, one being the free will defense/ argument. According to this argument, God must allow his creatures to do evil sometimes in order to promote free will. So even if God wants to prevent evil, he cannot because free will is very important. Free will successfully solves the problem.