For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
While God is demonstrating how bad it can get in a world that is certainly influenced by Satan, if, as Vincent states, “Humans (and Angels) sin, not because of the devil, but because of free will.” then Seth’s statement that, “Removing Lucifer would not have done anything to remove the “sin problem”.” is paramount.
This topic is closely related to the question: “What is the origin of sin?” In S. Michael’s answer to that question, he states, “All sin, therefore, must come from the creature, and the desire for evil comes from within the creature.” This is central to the “sin problem”. It is a defect in our nature.
We could get involved in a long argument about WHY God allowed such a defect, but that would be counter-productive. Rather, let’s focus on what it is He is going to do about it. He is not simply demonstrating how bad the world can get under Satan, nor is He demonstrating His right to universal sovereignty through His kind intentions toward those who love Him and know Him in a personal way. Neither of these venues effects anything to solve the sin nature that is found in all of humanity.
S Michael, in his answer to the question, “What is the origin of sin?” concludes that once God’s redemptive plan is complete, Jesus Christ will have destroyed the devil’s work forever (1 John 3:8). This is certainly true, but how?
It has to come through a change to our nature. God’s inability to lie, or be tempted by sin, will be written on our hearts (Heb. 8:10).
God has a purpose. We have all sinned, but he doesn't immediately destroy us. Everything created, God has a purpose for.