Its all about perspective
Okay,
How about this.... You are the cop (LEO) and you have a gunman shooting. You are going to engage and then you see another guy pull his gun too. Hmmm. Now as the LEO do you have one or two BG? Tough one to say depending on how he/she perceives you. You already know the first one is definitely a BG as he's shooting people.
In a split second assessment he engages an active BG. Now, what to do with you. Depending on the angle and how things unfold he may be shooting you next.
It's like a kid pointing a very realistic airsoft gun at a cop. He's justified to shoot you in all 50 states if the appropriate actions were taken and the child failed to follow or made threatening movements.
So, be aware of your situation.
Playing the WHAT IF's
If you can take out the BG right then? Hmm, tough one. I would. If not, let the LEO engage. If he's not able to then back him up. Let him know you'll be his back up as a law abiding citizen with a CCW. He'll appreciate having the help. How much he uses you will depend on the situation.
As to not engaging because it wasn't "my family or me" That's a total waste of time. If someone is shooting people in the mall and you didn't do anything... you have to live with yourself and may God have mercy on your cowardly soul.