You're phucking goofy... Whether it be culture, morals, religion or whatever, not everyone reacts the same way in certain situations. I couldn't disagree more with your assessment.
No, they don't. But
most people simply don't turn their back to someone who they view as a threat. If you're scared of someone, you don't turn your back or give him time to attack you while you go get a gun when you already have a perfectly good one in your hands.
But, let's just assume that this guy isn't like 99% of people who don't ignore something of which they're allegedly shitting their pants over. Unfortunately, the law is based on a reasonable person standard.
So, I can't tell the jury, "Sorry guys, I was really scared shitless of that 90 year old wheelchair bound lady who was screaming for help, so I shot her." Nor can I be blitzed out of my mind on drugs and try to explain to the judge, "Look man, you don't know how everyone reacts; whether I was on LSD or not, I personally thought that this five year old was aiming a cannon at me."
Those are obviously hyperboles, but my point is that the manner in which he acted was unreasonable. A person lying on the floor who's been shot in the head and isn't moving does not, to a reasonable person, pose an immediate threat to you. That is made even more clear when you see a video in which the man has the time and ability to turn his back to the "threat," walk away from him, come back, walk right past him, get a second gun, walk right up to the guy within reaching distance, and then shoot him five times.
Sure, maybe the guy was scared shitless and thought he was in danger, but you don't get off on self defense just because you claim to have been scared. The law uses a reasonable person standard so that we don't set free ticking time bombs in the form of over reactive gun slingers who think that someone's a danger to them just because they asked them for change.