The shooter was an off-duty security guard who decided to play police officer. Stupid. And tragic.
Unfortunately, it’s something we see now and again. An armed (or even unarmed) citizen decides to set some stranger straight. That rarely ends well.
Over 300K times/years it ends very well. According to the FBI that is the minimum number of times a gun is used to stop or minimize a violent situation.
The kids were stupid. If someone points a gun at you and you do not do what they say you deserve to get hurt. Uniform or badge is irrelevant. A gun was pointed at you and your defense is a piece of plastic.
Openly carrying any type of gun into a store is an invitation to intervention. If it doesn’t fit into a holster put it in a trash bag so as not to alarm folks.