As a guy who used to do armed security, this situation never really crossed my mind. Was the guard in a visible uniform at the time? If so, the cop should go to jail for murder.
How ever, it is a scary thought and you really do bring up a great point about carrying a gun. Concealed carry people do occasionally get sucked into a violent situation and deal with it, how then do arriving cops then sort it out with out killing more people? How do they on the fly figure out who is good guy and who is bad guys? It is even more complicated in the wake of Columbine, with officers now prone to run in and shoot first and think later. (They were heavily criticized in the wake of Columbine for not being more aggressive and waiting)
I think more training for sure. Taxes need to be raised and the mostly abysmal tactical training cops get should be greatly expanded. As well their should be some mandatory training for armed Guards. Maybe train them to drop all weapons and assume the felony position when cops arrive? Clearly something needs to be changed.