God didn't send Satan to wreak havoc on Earth.
Satan used his freewill to do it.
Scripture is clear that God sent Satan to Earth:
Revelation 12:7-12
7 Now war arose in heaven, Michael and his angels fighting against the dragon. And the dragon and his angels fought back, 8 but he was defeated, and there was no longer any place for them in heaven. 9 And the great dragon was thrown down, that ancient serpent, who is called the devil and Satan, the deceiver of the whole world—he was thrown down to the earth, and his angels were thrown down with him. 10 And I heard a loud voice in heaven, saying, “Now the salvation and the power and the kingdom of our God and the authority of his Christ have come, for the accuser of our brothers[a] has been thrown down, who accuses them day and night before our God. 11 And they have conquered him by the blood of the Lamb and by the word of their testimony, for they loved not their lives even unto death. 12 Therefore, rejoice, O heavens and you who dwell in them! But woe to you, O earth and sea, for the devil has come down to you in great wrath, because he knows that his time is short!”
It is also clear from this that the consequences were known. Satan was sent to Earth deliberately and God knew what the outcome would be.
And if God would have just destroyed him, who would have taken his place?
No one.
THAT'S THE POINT.
Why does the Bible need a bad guy? From an atheistic point of view, the answer is simple - the Bible needs some character to have conflict with God and the forces of good to account for the fact that there is strife in the world. But the problem is that God is so powerful there is no actual reason why he would let Satan continue running around causing trouble. God, being omniscient, knows what Satan is doing AND he knows how to stop him. God, being omnibenevolent, is motivated to stop Satan from doing what he is doing. And God, being omnipotent, is capable of stopping Satan from doing it. There is no plausible reason why God has not yet done it.
But that leads back to that problem: You
CAN'T say God has done it, because the second you do, people will see that there is still war and conflict and all those other things that are attributed to Satan, and then everyone will say, "Hey, I thought Satan was responsible for that stuff, but God stopped him. So if there's still conflict, then it must mean your claims were wrong! The religion you present must be wrong!" And then people would leave. So in order to prevent this, the only explanation is to say that God hasn't stopped Satan YET. And, of course, there's no good reason why that should be the case. And I've seen a whole bunch of outrageous leap[s of logic to explain why God hasn't done it, but none are ever convincing.
