It may have been inspired by being struck at the tragedy of Buddy Holly, Richie Valens, et al, but that doesn’t explain the lyrics at all. I’m guessing McLean must’ve been outraged at the happenstance of their death, and knew to redirect it.
I always took it as a "dirge in the dark" not only about the deaths of the musical icons, but also the death of America the Free and moral and decent. You could feel the changes in the air and know that it was not good.