Condition Yellow
You never know when the person next to you will decide that "now" is the perfect time to meltdown or self-destruct. IMHO, high population density in urban centers is a big contributor to feelings of depression, insignificance, and increase the desire to be noticed even for something horrible.
The video you posted has more to do with the nutjob's desire to control his environment than anything else. This is a perfect example of why living in Condition Yellow ever more critical.
If you are aware of your surroundings, and what people around you are doing, you have a much better chance to recognize a potential threat BEFORE it becomes a true emergency, and take action to remove yourself from the scene or develop a secondary response.
The absolute best course of action for the private citizen in a situation like the one in the video is to evacuate the area post haste BEFORE the battle starts. Street shootings are bad mojo for all concerned. IF you win the fight, the courts will probably have your behind for a long time, and a perfectly legal defensive shooting will cost you dearly in time and money. YMMV