To answer this question, we first need a little history review.
Satan didn't start out bad.
God created Lucifer (Ezekiel 28:13, 15), a beautiful, shining angel who was a leader in heaven (Ezekiel 28:14; Psalms 99:1). One day, Lucifer began to think more highly of himself than he should (Ezekiel 28:17), and he began to question God's authority in his life. What began as thoughts in his own mind turned into public murmuring among the other angels.
This murmuring became louder and louder, until ultimately, the Bible tells us Lucifer wanted to remove God from the His heavenly throne and sit there himself (Isaiah 14:13, 14). He fomented a rebellion that drew one-third of the angels to his side and caused a war in heaven (Rev. 12:7-9)!
Because of this, Lucifer and his angels were cast out of heaven. His name became "Satan." He found his way to the earth where he corrupted God's perfect garden creation and the two human beings who dwelt there. It has been misery for earthlings ever since.
It's reasonable then to wonder why God didn't destroy Lucifer and his angels instead of just casting them out of heaven. One wonders why God didn't just destroy Lucifer right at the beginning, when he started murmuring. Wouldn't the destruction of Satan have prevented all the suffering in the world today?
Lucifer started this controversy by attacking God's character. He asked aloud, "Is God's way the best way? Does God deserve to be God? Does God deserve our love?" All of heaven began watching this controversy intently, waiting for the answers.
What if God had “answered” Lucifer's questions by destroying him? Being destroyed for asking questions doesn't seem fair. Can you imagine what the other angels would be thinking? At least some of them would have wondered if Lucifer was right.
They might have come to the conclusion, "Don't question God, or He'll destroy you!"
Now, instead of worshiping God out of love for Him, they would worship Him out of fear. Many angels would be left with doubts about God's character. Yes, God could have destroyed Satan, but that would have left behind doubt and fear.
God is love. The only loving way to quell this uprising was to let it complete its course so the answer would be obvious to all. The best way to answer Satan's questions was not to destroy Satan, but for God to demonstrate why His way is the best way, why He deserves to be God, and why He deserves the love of His creation.
What do you think is better: To serve some one out of love or fear? Why? Do you agree with God that He didn't destroy Lucifer immediatly? Why or why not?