There are a couple of important things to note that it seems you guys have missed.
In the video, the girlfriend originally claims that the victim was "just trying to get his license and registration." She claimed it was in his wallet, which was what she said he was reaching to get. Yet, at around the 3:45 mark in the video, while the phone is on the ground, she is heard saying "he tried to tell you he was licensed to carry and he was going to take it off."
If "it" refers to his gun, which it seems to mean, it raises a very important question and possible scenario. Was he trying to take it (the gun) off like the girlfriend claimed he was going to do or was he simply going for his wallet like she originally claimed? If the former, and without the consent of the officer to do so, I can easily envision a scenario where the officer saw the victim reaching for the weapon to "take it off" as the girlfriend claimed.
Also, in the video, the girlfriend claims "we only had a little weed in the car." Possession of weed isn't an excuse for lethal force, but it does give a glimpse of character.