Why didn't God kill or destroy Satan?
It’s important to remember that Lucifer had challenged the fairness of God’s government. He had told lies about God. The idea of telling a lie had never entered the minds of the angels. Naturally they didn’t grasp all that lay behind Lucifer’s deceptions.
If God had destroyed Satan immediately, the angels would have begun to serve Him from fear rather than from love. This would have defeated the very purpose He had in creating beings with the power of choice in the first place.
How did anyone really know God’s way was best? No one had dared to try an alternative. God gave Satan a chance to demonstrate his alternative system. That’s why he was given an opportunity to make his pitch to Adam and Eve. This planet has become a testing ground where the character of Satan and the nature of his kingdom are contrasted with the character of God and the nature of His kingdom. Who is right? Who can we ultimately trust? These are the issues which “the great controversy between Christ and Satan” must resolve.
So deceptive was Lucifer, that before all the universe could be fully convinced, the nature of his alternative had to be fully exposed. It has taken time for everyone to really grasp how disastrous Satan’s alternative actually is. But eventually everyone will see that “the wages of sin is death” and that “the gift of God is eternal life in Christ Jesus our Lord” (Romans 6:23).
Every being in the entire universe will then agree with the declaration:
Conclusion: God's perfect plan
After everyone understands the deadly nature of sin and the destructive nature of Satan’s philosophy, God can destroy Satan and sin. He will also have to destroy those who stubbornly resist His grace and cling to Satan’s alternative.
God is just as anxious to resolve the problem of sin and suffering as we are to have Him do so. But He is waiting until He can do it on a permanent basis, and until He can both preserve our free will and prevent evil from ever appearing again.