I think once Giffords got shot in the head he might have had an idea that Loughner might be a danger to others.
You said before he started shooting. Once he started shooting, everyone had to keep their head down until he stopped to reload. Many of those who couldn't make cover got shot too.
I am not arguing for the repeal of the 2nd amendment. I intend to get my CHL and, once she get her real estate license, Miss Limey will most definitely be packing. But the argument that more guns means less people getting shot is a fallacy. If it were true, Texas would be nearly devoid of gun deaths. The issue is what armaments are available, and to whom.
Loughner had a banana clip that had been illegal a few years earlier until the assault weapons ban was allowed to expire. He was tackled as soon as he stopped to reload. Had he had to stop to reload after 10 shots, instead of 30, fewer people would've been shot/killed.