People downvoting you for a reasonable question?

We still don't know what happened, but... If he grabbed a knife after he opened the door of the car the officer behind him had only a split second to react before he might have gotten stabbed multiple times. If you do have to shoot someone who has a knife a single round, or two, or even three, frequently fails to do the job. Even if they are mortally wounded by several shots, someone with a knife might still have enough gas in the tank to kill you before they succumb to injuries. Because of these quite indisputable facts (for which you can find ample video evidence) police are trained to shoot until the threat is gone, i.e. suspect drops or they are clearly no longer a threat. Its rough, and looks very bad on video, but its rarely as callous and one-sided as it at first seems to people unfamiliar with the nature of such close-quarter confrontations.