Originally Posted by GFED

"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."

https://www.bibleinfo.com/en/questions/why-didnt-god-destroy-satan