Is it that some folks are just so self centered that they can intentionally allow others to be hurt or killed simply because they don't feel they should get involved? Is it that many lack confidence in their ability to use their firearm except as a last resort? So often there are references to"trainers" who recommend that people are better off hiding in a hole until the boogey man ferrets them out and they must act or die.
IMO, a lot of this is not that people are self centered. It's vicarious liability. Instructors tell people to stand back and be a good witness, because they don't want to train a George Zimmerman and be held liable.