Today we think about an event that was not only the result of evil, but also the reason for allowing evil to exist in the first place. That event is the revealing of the perfection and beauty of God's grace and righteousness through the demonstration of both on the cross:
For all have sinned and fall short of the glory of God, being justified as a gift by His grace through the redemption which is in Christ Jesus; whom God displayed publicly as a propitiation in His blood through faith. This was to demonstrate His righteousness, because in the forbearance of God He passed over the sins previously committed; for the demonstration, I say, of His righteousness at the present time, so that He would be just and the justifier of the one who has faith in Jesus (Romans 3:23-26).
Why did God allow evil to come into this world? Why not create everything in the state of perfection we will be in after this world comes to an end? I think the answer is that God had a goal in mind that is greater than the suffering, and that goal is the revealing of Himself to His people so that we will be able to fully express our pleasure in Him through worship, enjoying Him for an eternity.
In other words, we experience suffering and sin so that Jesus could die on the cross for us.
That might sound backward to you. But if God's greatest goal is to reveal His perfections to us (a greater goal than our temporary comfort on this Earth), then in order for us to know God's grace, His mercy, His power, His justice, His righteousness, His love, and our need for Him, He allowed sin into this world so that we could see and experience Him in these ways. And the pinnacle of this demonstration of Himself happened at the cross.
Evil isn't necessary for God's goodness to exist, but it is necessary for God's goodness to be revealed to us. I always think of the heroes of United 93 as an illustration of this. Before 9/11, the people who would soon give their lives on that flight went about their daily business doing ordinary things. They had already developed the character, invisible to us, that would direct their actions on that tragic day, but the depth of their self-sacrificing courage wasn't made visible to us until evil led to the expression of it.
In the same way, God was God before sin was in the world. He was always full of grace, but without our sin, would we have known it? He was always just, but without judgment, would we have seen it? Would we have ever seen a love that seeks out enemies if there had been no enemies? Our knowledge and appreciation of God would have been forever stunted in a world without the cross.
In love He predestined us to adoption as sons through Jesus Christ to Himself, according to the kind intention of His will, to the praise of the glory of His grace, which He freely bestowed on us in the Beloved (Ephesians 1:5-6).