To the original poster.
So instead of leaving before he/she/it got there you got held up in your timing browsing at the birthday cards and then you finally decided to head to the cashier at the fateful moment.
So I ask you..What would you have done??
Tactical patience. Based on the video (I only know what everyone else knows) I did not see an imminent threat to life. I am not of the mind that my firearm is only to be used in defense of myself and my family. I am willing to defend an obviously innocent third party as well. Nonetheless, this appeared to be a robbery and nothing more. Tactical patience demands that I seek to place myself in a position of advantage from whence I could exact a violence of action to overcome the deadly threat should one be manifested, but only if it is manifested.
Arguably, a robber presenting a weapon (or what is implied to be a weapon) does constitute a deadly threat. The robber in this case had the "means and opportunity." I think the law is pretty clear that the third prerequisite of "intent" is met as well. A criminal, during the commission of a felony, not only possessing a firearm but pointing it in the direction of others is more than enough action to articulate intent.
Based on that information, I do feel that I (or anyone else) would have been justified in the application of deadly force at any time during the immediate commission of this crime. That does not mean it would have been the prudent thing to do. Like I said, I would have observed until I felt like there was no other option, but not a moment longer.
Unlike some, I would not simply ignore the whole situation. 10 years in law enforcement has ingrained in me an unwillingness to simply do nothing, as long as it does not endager my family or others. I am ok with a little self exposure that is mitigated through proper tactics.
That being the case, I would have followed the subject while calling 911. I would have attempted to ascertain any getaway vehicle information or at least a direction of travel. Maintaining 911 communication, I would also be inclined to follow in a vehicle (from a safe distance) or even on foot if that was the suspect's mode of travel. I would not make any attempt to detain the subject or hinder their escape. Nonetheless, should I be discovered and a threat presented itself, I would deal with that accordingly either through a graceful exit or strong defense, situation dependent.
To be clear, I do not recommend taking any further action than the initial observation and a report to 911. I am simply speaking from a unique perspective of what I know I would do and how far I am willing to go, combined with 10+ years of law enforcement experience that enables me to take some reasonable level of action. I do not believe in a willful escalation of the situation. I do believe in allowing on-duty police officers to do their job. At the same time, I cannot personally do nothing at all. My training and experience has allowed me to do more than that while minimizing risk and my values demand it.