We should note there are two types of evil - natural and moral. Moral is easier to understand, it is the result of our sin through exercising our free will. Natural includes earthquakes, floods, disease. Why is that allowed? To make us better persons? To teach us to help others? It is due to Satan creating havoc? It is an unavoidable result of God's natural laws working consistently? Often we use the IJDK explanation - "I Just Don't Know."
But what is the greatest evil that ever occurred? Isn't it that the infinite God, Jesus, came to earth and was crucified to redeem sin? And yet God changed the greatest evil into the greatest good - forgiveness for humans and eternal life. If God would do that, I certainly will not suggest that, because of evil, God is not all good!