Imagine yourself as a juror. I think most of us are doing just that.
With all these big question marks left floating, could you say David A. Westerfield should be put to death because you are convinced that a) he did it and b) no one else could have and c) the investigation was positively thorough?
I think I would have to vote, "Not guilty" because there are too many doubts. Thus I have reasonable doubt and solid basis for having that doubt.
I don't want a killer set free and in the back of my mind I would always pray that I made the right decision. I would be equally worried about condemning an innocent man. At this point the latter thought would prevail.