It seems to me the shark obviously came around to investigate the diver a number of times, entering into close quarters. It's not credible that he was chasing it around, since the shark can easily move much faster than a diver, and I think even a casually swimming GWS would be hard to keep pace with if heading off in a straight line.
It's been said that a number of shark attacks are thought to be 'test bites' where the shark bit a human to see whether he or she was food, decided not and swam off. Problem is, getting 'taste tested' by a large great white can leave you hemorrhaging to death, though being uneaten must surely be a comfort (if you're in a frame of mind to care by that time).
While the chosen music score affects the viewing experience, if that were me, I'd be thinking along the lines of 'If this thing kills me, I hope somebody finds the camera so people can figure out what happened.'
I think shooting the shark in that circumstance was the right thing to do.
Richard.