Why Would a Good God Allow an Evil Devil?

It was necessary to allow the universe free choice. If God killed the devil when he first rebelled, every other being would serve God from fear of the same fate if they didn’t obey God. God didn’t want that, so He had to allow the devil an opportunity to demonstrate his principles and what they would bring, and we are beginning to see it!
There was war in heaven and Satan was thrown out to the earth and given access to Adam and Eve at the tree in the garden. Adam and Eve failed the test of loyalty to God by accepting the devil’s lies (Genesis 3) and a night of woe came upon this planet.

Adam and Eve would have died for disobedience except the plan of salvation promised them restoration if they recognized their mistake and lived a life of repentance toward God with Christ coming to pay the penalty for their sins.
Christ came to show that God’s law was fair—it could be kept and God is merciful—He allows His life of perfect obedience to be credited to anyone who appreciates the Savior’s death for him or her and seeks to live in repentance.
The devil can tempt us to sin, but it is our choice and will to do it that makes it wrong to us. Nevertheless, the gospel (good news) is that God gives power to overcome sin through the Holy Spirit given to us by accepting Christ’s death for us, and we need to see that our sins drove the nails at Calvary.

Asking God’s help for whatever our problem, we can be assured that He will help us, but it is not always as we wish or expect. Nevertheless, it can work for good, just as a drug addict may suffer withdrawal from what he brought on himself.
Multiplied seven billion times, that’s where the world is today—and Satan should get the blame, not God, as most people do their own thing. This is a brief explanation.
For a better one, the reader may consider the opening chapter of a biography on the life of Christ that for many years was most recommended in the Library of Congress. (They no longer recommend any, but just give a list.)

Here’s a link; the first several pages are background to the above. Keep reading if you like it or select some favorite chapters like the closing scenes of Christ’s life starting on page 642, His last night, for example. http://www.thedesireofages.com/The_Desire_of_Ages_text/DA.pdf