Is there a legal requirement to have a reason to believe the gunman would actually shoot (assuming he wasn't further provoked) rather than take the drugs and leave? {We can assume he turned his attention and gun to the security guard b/c the security guard moved rapidly from his original position.}
Would this be the equivalent to shooting someone who is in your house without making any attempt to avoid shooting? i.e. You see someone in your house, "see" (or believe you see) a gun, and just shoot them with no hesitation.
The videotape in this question seems like it could be a liability, no?
[I'll agree that once the guard began to act, the risk to everyone went up quickly and shooting without hesitation was probably a good idea. However, I can see a liberal prosecutor trying to find a way to blame the security guard for the death, saying perhaps that he himself raised the stakes by making a move. Now, I couldn't tell if there was audio on that security tape to hear what was said which may influence the judgement.]