You don't know that he was going for a weapon. You're speculating. You are further speculating the cops made the same assumptions you're making, and ignoring the likelihood that the guy seen walking to his car where his kids were waiting was trying to exit the confrontation, not escalate it.
Even if that's what he was trying to do, he escalated the confrontation by reaching into the vehicle. Those police officers did not want a repeat of the Daniel Clary clusterfuck. Neither would they want a suspect gaining control of a 2 ton lethal weapon, even if there was no other weapon inside.
Its a clusterfuck when a police officer gets harmed. But when black people get killed, it doesn't matter, because their lives don't matter. That is really what you are saying. And that is what the protests are all about. Any potential risk to the police, however small or theoretical, has to be eliminated, even if that means killing another human being. If an officer cannot tolerate even the smallest risk without killing somebody, maybe they shouldn't be a police officer. Millions of policemen all over the world continue to do their jobs every day, dealing with drunk and belligerent people, without drawing a gun and killing anyone.
It seemed to me that that he was trying to get to his car, perhaps to reassure his kids, perhaps to disengage from the confrontation. Why should we assume that his intention was to harm anybody? The only people who pose a risk to the safety of people at the scene are the police, with their guns drawn, waving them around. And they follow through by shooting a man seven times in the back. If you don't understand why someone might see this as being fucked up, you are badly broken.