One thing that stuck out to me was "he place the BB gun on the shelve with other guns". Why was a real loaded gun just sitting on the shelve to start with? All gun store I know of always double check to make sure nothing is loaded. They may have a loaded gun but it is in a holster on their hip not just out for anyone to pick up.
Something does not seem right.
This was my first thought too. I'm one who usually gives the benefit of the doubt to people and not jump on the, usually, outlandish alternate theory. For me, this sounds like the shooter wanted to kill the other guy and this was his way of "getting away with it".
"Check the tower logs for Christ's sake!"