Posted by Seaman: I would order him to stop and kiss the pavement, then call in the cavalry, and you wanna know I wouldn't take my eyes off the BG excepting glances to my six.
I am not sure why you think it would be wise to issue such an unenforceable command without having given any indication that you are armed, or why it would be to your benefit as someone who is not a sworn officer to try to hold the person in the first place.
I'm sure you are well aware that you may not shoot simply because he chooses to not comply.
I'm not sure you have considered the legal risks, criminal and civil, that you would face should he turn out to be unarmed and dispute your account of the encounter.
And, of course, the tactical risks--getting shot in the back or side by his accomplice while your attention is focussed on him, "glances to your six" notwithstanding.
Not the best idea, in my view.....
Whether and when one should draw in such a situation is going to depend on a lot of things and will require some quick judgment.
If it turns out that you were in fact in imminent danger and did not draw quickly enough, the outcome could be disastrous.
If you draw, and the danger dissipates with no shots having been fired, great.
If you draw and shoot and he drops a weapon, you have defended yourself against the immediate threat, and your action will then be the subject of an investigation.
If it turns out that he was unarmed and bluffing and you draw, or draw and shoot, you had better hope that you can produce some evidence of justification, and that you are not faced with unfavorable witness testimony.
The outcome for the OP was just fine.