This is a case where I'd say you don't "rise to the occasion", you default to your level of training. I can't say what I would or wouldn't do without being right there, right then. I tend to trust my instinctual reactions to stressor situations because it's worked well for me so far. I've aided people in distress and I've yet to be forced to shoot anyone. I've also pulled back and called 911, but usually before things got to the point of life or death.I think the more relevant question is how does the bystander know it’s the right guy?
Also I’d like to point out how crazy it is to pull out a gun during a police encounter like that. More police show up while you have that gun out and you are now the prime focus of their actions.
I'd be interested to see the CCTV of this event because you know the casino probably has it from multiple angles. Only then would I risk making an assessment.