I cannot see in the video what the guy was doing well enough to judge whether they were right to shoot him or not.
I can add two things:
First, at ten feet or so a guy with a knife is a lethal threat. He can fatally stab someone in a few seconds at that distance.
Second, once the cops decide to shoot they are trained to finish the job by shooting until you are down and you stay down, which is why they shoot so many times once they start shooting.
So the real question is whether he really did something to make himself an immediate threat. The rest is protocol.