One of the victims was a chauffeur on his way to pick up a bride and take her to her wedding. Clearly he at least was just travelling perfectly legitimately along a public road with no intent of stopping to watch the air display. Therefore a completely innocent member of the public died as he went about his own business as a result of this incident. He was not trespassing, he was in a public place, and there is strong evidence to suggest that he was a completely innocent victim of circumstance. How many other people were similarly innocent and just happened to be travelling past the display? I would suggest the majority. Those parked up and outside their vehicles had a chance to make a run for it as events unfolded.
Easy answer to preventing similar incidents, ban all display flights over and in the vicinity of any inhabited or transit areas. I don't expect that anyone really wants that to happen.
Therefore it is probably best to leave it up to the organisers, pilots, and maintenance crews to make such displays as safe as possible in the knowledge that they will collectively and individually face the full force of the law if something goes wrong.
It may take only a couple of moments inattention or misjudgement to contribute to a tragedy, but I think the public expect and are entitled to expect display pilots to be at the top of their game and the best of the best. Standards that were singularly lacking in this case, in display planning, in aircraft maintenance, flight discipline and piloting skills.