I found this link (
http://bubbaworld.com/2007/09/04/shooting-speeders/) dated from the time of the original incident which quotes the Rogers county sheriff as saying both men (Abshire and Case) may have used alcohol.
I don't know how accurate that is; but it might help explain the DA's actions when you add in the second shot to the back and the two conflicting stories about how it got there.
I would really be interested in seeing the transcripts from the trial. It sounds like there is more going on here than any of the news stories have related.