This whole situation was a tragedy. But tasers don't cut it for preventing imminent stabbings. Tasers don't have the capacity of a pistol, so if he misses he has to fumble for another weapon. If he hits the taser could get snagged on clothing and not deliver a charge effectively. And sometimes even when used perfectly the taser just doesn't work, you can watch videos where people get tased and just don't react. A gun, on the other hand, is way more likely to stop the aggressor before they harm anyone. If there is an imminent lethal threat, lethal force is justified. If someone has me pressed up against a car ready to stab me, I do not want the cop to use a taser to save me.
And shooting the hand, really? Sure he's a marksman, but the hand is a small target and moving fast (swinging a knife). I think mass media has made guns look way easier to use than they are in reality. I've shot pistols before and even hitting a stationary target at that distance is tougher than it looks. Sure, he's a trained marksman, but you're asking him to hit a small, fast moving target in a life or death situation. He's a good rifleman, not John Wick.
I feel like people who object this shooting are either: unfamiliar with how guns, tasers, knife violence work. Or have a visceral bad feeling when they see the girl get shot, and thus immediately assume that the shooting was bad. To be clear, it's good that you feel bad seeing a young girl get shot and killed. But that doesn't mean that it was unjustified in this specific instance.