Would be really interesting to see statistics about how many armed robberies end with one or more dead if only the robber has a gun and knows that he doesn't have to use it if nothing crazy happens or how many times people die in a robbery where the robber and the "defender" has a gun.
I would bet that not only more robbers die when both have a gun but also more people who get robbed or see a robbery and who try to play the saviour. I mean, sure, sometimes the robber will really kill someone and then nobody can do anything if only the robber has the gun but more often will the robber kill someone if he feels unsecure and thinks that he has to kill someone to get out of the situation.