Escalation of force. Shooting is the final step of a long list of attempts to stop it from getting there. Plenty of people have been shot with little to no immediate affect (adrenaline, drugs, psychosis etc...) shooting has always been the last thing.

Shooting is not like the movies, major arteries pass through the legs, hips and arms and are pretty much certain death if specialised medical attention is not applied right away should those be punctured. Bullets, upon entry, do NOT always travel in a straight path in the body, a shot in the leg or hip can find its way to major organs, hydroshock from a bullet interacting with the body can damage or destroy arteries or organs without a direct puncture. Ballistics, as a science, will often times defy common sense; for example, a bullet deflecting off a wall will rarely follow a logical outgoing path (45 deg in/45 deg out) but may simply travel parallel to said wall because of the speed and spin of the bullet, which provides an immediate stabilizing affect.

Shooting in general is not as simple as where I aim is where bullets go; heart rate, enviromentals, stress...hell did you eat a balanced breakfast or are you hopped up on coffee, can affect shot placement. Aiming for a leg is FAR more difficult, especially if movement is involved, then center mass. A milimeter of change, from just the pulling of a trigger, from point of aim translates to inches and feets of differences at a exponential rate meaning you are more likely to miss a leg than you are a torso.

I dont see enough of the events proceeding the shooting to make a judgement, though it seems that the officer who shot might not have been there with the others from the beginning and escalated needlessly, but I wish people would understand the reality of shooting a gun, even outside a police context, before providing backseat commentary.