Actually it doesn't mean that it had to have happened before God created Adam and Eve. One thing people don't realize is that we have no idea how long Adam and Eve lived in the garden before they sinned. It could have been thousands of years. Or a month. We don't know. We don't know if Adam's age in the genealogy starts from the fall or from the day of creation (why would they have been keeping track of how old they were if they were never going to die?) Also to say that Satan was "cast out of Heaven" is a bit of a misnomer - it makes it sound like he can't get into Heaven - he's there now - accusing believers left and right, like he did to Job. But yes, 1/3 of the angels sinned and fell pretty early on. Thankfully though, God chose to allow redemption for mankind.