Emotions removed. Six guys ontop of a guy who never drew a weapon, have his hands down, removed the weapon already.
Did they think he was able to operate a gun using his sphincter?
Zero justification. You are way offside on this one.
His hands were free and waving around, they knew him to have a gun and were shouting 'gun' and then a gunshot rung out.
In a chaotic situation like that people don't get the benefit of poring over slow motion zoomed in stabilized replays of videos from 3 angles each time in the heat of the moment, they had a split second to decide what to do when they heard a gunshot.
Think for a moment what you are saying. "Zero justification". After hearing the gunshot. In that moment they aren't trying to guess how he's operating a gun or whether its the sphincter-operated ass gun Key and Peele would before cutting people's faces off with a serrated blade. All they know is a shot was just fired inches away from them.
That is of course hinged on a theory, but it seems the best explanation so far.