Reports like this rarely can get the facts straight. Regardless of who shot first, the fact is that an armed robber accosted a victim. A "person" put in fear of their life can defend themselves.
Most gunfights occur within ten feet of each other, last only a few secondsand usually about twenty rounds are fired. The stress of shooting at a moving target that is shooting back at you is enormous.
There are no hard and fast rules for those who have never experienced that kind of danger, except to be as much aware of your surroundings as you can. Open or concealed carry won't mean much if the bad guy approaches from the rear or out of your sight, or when you are distracted elsewhere. The sooner you can detect a potential threat, the better.
Rmember, nobody can predict what they would do in these situations. However, the more you practice, the better your chances.