Focus on his intentions would have given me the element of surprise. I carry a concealed weapon every day (with the proper permit, of course). I am required to qualify with my weapon, I am trained, and I practice often. In short, had I walked in on that situation, I would have been the right man at the right time. And let us suppose that the timing would have allowed for me to see the situation unfold, and that I was able to drop the assailant at the instant of his attempt.
Herein lies the quandary: Assailant raises his weapon, I draw mine somewhere out of his focus, holler for him to drop it, he does not, and I drop him. In that instant, we have two officers reacting, presumably drawing their weapons, to an ambush with someone shot and someone else holding a smoking gun. Yesterday I was wearing jeans, sneakers, and a green hoodie. Take the information you digested reading this paragraph and condense it all down to 1.5 seconds, the time you have to make a decision about what to do next. Amplify that information with the knowledge that your uniform makes you a target, your ears are ringing from the gunfire, there is panic all around you, and your occupation instantly puts you in charge of public safety. And then put a gun in your hand. What would your reaction have been? And know that your answer is really only valid if you can say what you have done in that situation because you have been in it.
I am confident in the intentions of the two officers who were on their break, enjoying a cup of coffee. I pray that in that situation, I am able to identify myself to them (again, everyone's ears are ringing and you don't know what anyone can hear or perceive when you say you're one of the good guys) before they drop the guy who just shot someone. But if, in the pandemonium of that situation, they neutralize the chaos of gunfire by shooting the shooter (you know, the one who happened to be dressed like the Unabomber), could you blame them?