You might think so, however you would be mistaken.
OCing that guy would just get you shot in retaliation, and maybe everybody else on that sidewalk if the shooter had impaired vision due to the OC working at all the way it was designed and the bad guy decided to do what many armed robbers and gang bangers do, spray-and-pray, when they decide to shoot.
As for shooting that guy, read up on Garner vs Tenn., then do some reading over at the Force Science Research Center ref human reaction time. I could easily articulate a fear for my life, a need to apprehend that suspect before he robs/shoots/kills anyone else (cop think, being a career cop and all), that he was obviously a well practice professional criminal with experience in robbing people who was willing to go to guns over what could have been a simple petty theft. Announcing or otherwise doing the "drop the gun" thing very often gets good guys shot as it gives the bad guy choice of first move.
Although, out of reflex I would have likely jammed his draw and then disarmed him or shot him repeatedly from a high 2 position, so it probably wouldn't have gotten to the point that he was walking away. Since I have actually pull off such disarms on the street during the course of Terry stops, on more than one occasion, I can speak from experience there.