” reached for something that did turn out to be a weapon.”
But at least something should be in the hand yes?
Otherwise, with your reasoning, if a cop approaches and your hands are not in view, when he asks you to show them, when you do, since there could be a gun in it, it would justify shooting.
An impossible condition to meet.
Then at 4:26, when he was in movement as instructed and closer to the cop, he AGAIN reached behind his back and brought his right hand forward very quickly. This spooked the cop into the split second reaction to shoot.
Reasonable people should only expect two thing in situations like this. 1) The guy/perp should not do anything that would make the cop fear for his/her safety and 2) The cop should not shoot until the last possible moment when he/she does fear for his/her safety.
One thing is clear the guy/perp repeatedly caused the cop to fear for his safety even after being repeatedly instructed and warned against moving his hands his back. He didn't even try not give the cop reason to fear and shoot.
The cop did at least try, since he did allow the guy/perp once to reach behind his back with only a warning. But he did not have the nerves of steel and lighting reflexes to hold back when the guy did it again more quickly and when in movement more closer to him. And was spooked into firing - even before seeing something in the guy's hand. (Many cops have been shot and killed waiting longer.)
If all cops were required to have nerves of absolute steel and lightning fast reflexes society would be able to hire only a minuscule fraction of the police force today's crime ridden society requires.
As I said before, "Not the best outcome but an understandable outcome." But as I also said before "the officer did sound like aaahole" and many will judge him only on the basis of that.