If I had been on that deck watching the interview being made, I'd like to think I'd be alert enough to spot the bad guy and his gun and realize what's about to happen. And I also would like to think I'd have the courage to immediately act, likely drawing down on the bad guy. But I won't know until I'm actually in that situation.
I've watched the video several times and, there are those 23 seconds between the bad guy first pointing the gun at the reporter, then waiting for the camera man to turn the camera so as to get the murder on live TV, then opening fire. Twenty three seconds is plenty of time to draw from any kind of concealment, see if there are innocents in the line of fire, etc. There can be little doubt that the bad guy had deadly intent - you don't point a gun at somebody out in public like that just to see if the sights are still working. I'll offer the opinion that any reasonable person, seeing what is shown in the bad guy's video, viewed from almost any perspective on that deck, would easily conclude he had a "reasonable belief, based on objective facts" that the bad guy had just committed a serious crime (I'll leave out brandishing and just call it assault - the real and armchair lawyers here can offer their assessment as to the legal specifics of the crime) and the innocent reporter, camera man and interviewee were in imminent danger of a further assault likely to result in serious bodily injury or death. I don't see in the videos the kind of expanse of "gray" that often obscures a bad guy's intent and makes it hard for a good guy to decide what to do. Turning all this over in my mind, I can't think of any reason for a good guy with a gun to hesitate.
If only there had been an alert, armed good guy on that deck, looking in that direction, and resolved not to let evil triumph by doing nothing.