If God knew Satan would spoil his creation, why did he not send him directly to hell after his heavenly rebellion? Why did he let him hang around and make life difficult for everyone?
We will have to ask God these questions when we enter heaven.
He certainly knew Satan would tempt Adam and Eve. He knew they would make the wrong choice. He knew their defiance would damage his perfect system. He knew Jesus would have to die as a result of their sin.
We can only speculate why God has allowed Satan to roam the earth.
The Bible says God demonstrates his love for us in that while we were yet sinners, Christ died for us. This demonstration may be for more than just our benefit. God may want Satan to watch his love overcome evil, and his power defeat death.
God may want Satan to witness the totality of his defeat, so the futility of his heavenly rebellion haunts him forever. God may consider this eternal torment to be an integral part of Satan’s just punishment.
Similarly, the Bible indicates that in the next life, unbelievers will perpetually agonize over their missed opportunities to be reborn spiritually in this life.