Q. After Satan’s fall, why did God allow him to deceive Adam and Eve, and to continue to run rampant throughout the earth destroying lives when God could have simply cast him into the lake of fire from the beginning and have it done with, sparing mankind (including Adam and Eve)?
A. Like the angels, man was created with free will. God specifically told Adam and Eve not to eat of the forbidden fruit. In listening to the serpent they were consciously disobeying Him. Remember they had no sin nature at the time so they could have resisted. Therefore God did not “allow” them to be tempted.
If there was no evil in the world we would not have free will. We could only do and be good. Because of Satan we can love God and live in a manner that’s pleasing to Him by choice. This makes our love meaningful to Him. Otherwise we’d just be mindless robots.
Also, though the Bible only hints of this, I have concluded that had Satan not rebelled against God, mankind would not exist. When you get right down to it, there’s really no purpose for our lives other than to choose which one of them will be our God. There’s nothing we have that God needs, and as it says in Job 35:6-7, our righteousness doesn’t help Him and our sinfulness doesn’t hurt Him. Our behavior only impacts us and those around us.
In short, even though God knew about it in advance and wasn’t taken by surprise, I think the entire 7,000 years of the Age of Man was His response to Satan’s rebellion.