If he had time to say "You need to put your gun down," he had time to draw his own gun and fire several shots.
In retrospect, it appears that's the case.
The armchair quarterbacking in this thread (and others) is strong. Armed confrontations do not occur in the internet analysis spacetime where there is Max Payne-style time dialation complete with Prince of Persia-style time-rewind. In this particular shooting, the actions taken (whether they were deliberated, instinctual, reflexive, or whatever) resulted in a sub-optimal result. That's all we have to work with. IN THIS PARTICULAR encounter, his course of actions did not work out for him.
In another universe, he chose not to put his gun away, and got shot by an off-duty cop or other CCW. The Internet Armchair Quarterbacks (IAC's) decry his lack of judgement in 'walking around the mall with a gun out when there is no target in sight'.
In another universe, he did NOT pause for a split second when he was confronted by the shooter, and did a mozambique RFN. In that universe, the shooter turned out to be an armed citizen who went out to his car and got his trunk gun. The IAC's decry his idiocy at not verifying his target.
No matter what, the IAC's have something to criticize, which makes them feel better. But the real world is like one of those choose-your-own adventure books, only you can't cheat by holding your last place with your pinky finger so you can go back when it turns out you did one of the 90 actions that end the story and not one of the 5 that result in victory.