The bystanders who saw it are almost unanimous in saying they did not think he was a threat. Quite the contrary, there are multiple witnesses who saw him peacefully exit.
http://www.lvrj.com/news/man-did-not-pull-gun-on-police-at-costco—lawyer-says-98279344.html
But the 72-year-old man, in addition to another witness reached Monday, said they did see the weapon and did see Scott reach for it.
Likewise, if a person has a gun in their waistband and makes the decision to draw and fire it, they automatically have the advantage over someone who must process that threat and respond. Thus, if officers wait until the gun starts to come up, it's too late.
If you reach for a gun, you are in the midst if initiating a deadly force encounter. We don't judge incidents with 20/20 hindsight. We judge them from that moment in time, from the perspective of a reasonable officer.
Thus, the only thing that we examine, is whether it was objectively reasonable to fire at someone who reached for a weapon (according to two separate eyewitnesses) after being told not to move.
Remember, the police in that situation do not get the benefit of knowing all the extra information that you get to examine after the fact. They know that they have a guy with a gun in front of them, and he reaches for the weapon. That's it. Make an objectively reasonable decision in less than 2 seconds.