I am the only person I know that holds the view I am about to describe
And I say that humbly, not with pride.
Most likely, I am wrong.
I believe that God created a being whose purpose was to test humanity. Tests can be beneficial. We can use them to learn and grow. Tests are morally neutral. This testing angel tested Eve. She failed the test. Her failure created a test for Adam. He failed too. All of creation was impacted by the failure of those made in God's image, even the testing being. The failure and subsequent corruption of human beings led to the corruption of the testing agent. His motive for testing humanity changed from a desire to obey His Creator to a desire to deceive his subjects. He started to want humans to fail the tests he gave. The more humans failed, the more corrupt he became. His role of testing changed into one of temptation. The Fall of Man led to the Fall of Satan.
This theory solves a long-standing conundrum I have had. Why did Satan fall? I know why Adam fell: Peer pressure from the world (Eve). I know why Eve fell: The presence of a tester (Satan). I didn't know why Satan fell. It seems to me that there are only 3 factors that lead us to sin.
1. Our corrupted/fleshly desires (sin nature, however understood)
2. Our corrupted/fallen world (peer pressure)
3. The tempter (and/or any 'staff' such a being may have)
One or more of those elements in always present when we sin. But none of those elements were present in the case of the Tester (unless he tested himself?). What could have possibly motivated a good being to go bad given the absence of these three factors? I can think of no such thing (though, as I said, I could be missing something).
But if Satan was performing a God-given and morally neutral role in testing Eve... and if Eve failed the test, then Satan could have subsequently been corrupted by the impact of human sin on creation. His role became distorted just as the created order fell out of whack. It was, and is, creation gone wild.
Just a theory.