Before I say anything that might be misunderstood, I think this woman should be found and charged. I am also a firm supporter of the second ammendment; however, if we did as you suggest, people being what they are, I could just see the shootouts in the Walmart parking lots. I don't think we want to go there. While I believe there is a time for citizens to take matters into their own hands, at least temporarily, I really don't think we need a bunch of vigilantes running around playing cop.
Most States respect the right to carry. The scenarios you pose and fear have not occurred, nor are they likely to.
I live in an upmaket north Atlanta suburban edge city. Generally a really safe area. Our local Walmart parking lot had a shootout 2-3 years ago. Carjacker pulled a gun on a guy getting into his car. Guy pulled his own, shot the carjacker in the neck. The carjacker lived, but probably won't be carjacking again soon.
I talked to a cop from the precinct at length, the citizen was shaken up by the experience, and naturally so. He was reassured by the cops, who were high-5'ing him while he was at the precinct making his statement.
So that citizen shouldn't have had a gun, eh?