There are story after story of where they do so your assertion is flat out wrong. Go figure, you're wrong about most everything.
The exceptions make the news. not what happens normally, which is nothing. it if bleeds, it leads...
It happens often enough for reasonable people to know that it is capable of happening, such as to put one in fear of it happening to them when some asshole starts waiving a gun around and shouting demands. That very act implies that the perp is willing to use deadly force against the people in the store he is robbing.
The man who killed him, wasn't being robbed hence, he cannot claim self-defense, more than likely. Regardless, he will be sued. The lesson here, don't play the hero unless the shit really hits the fan...
This is the legal standard. This is from North Carolina but is the standard almost everywhere. Wow, who woulda thunk it! You're wrong again!
"In protecting a family member or another person, you can only use deadly force (i.e., your handgun) if, under the circumstances, the family member or
other person would be legally justified in using deadly force to protect himself or herself, i.e., to save the person from imminent threat of death, great bodily harm or sexual assault."
http://www.martincountyncgov.com/_fileUploads/forms/Concealed Handgun Permits and the Use of Deadly Force QA.pdf