How about if they atacked you, split your lip, broke your nose, and have you flat on your back and are repeatedly slamming the back of your skull into the sidewalk? Would you shoot?
That's why cops are taught minimum amount of force necessary. You may be getting beat up one second and then get the upper hand the next if you have the upper hand and use deadly force you will go to jail. even though you were almost beat to death a few seconds earlier.
Maybe where you live. The Albuquerque P.D. does a darn good job of handling difficult cases, but they are not required to allow themselves to get beat up. They don't go to jail if they shoot somebody though they are suspended with pay until an investigation is completed. Currently our new mayor is having a fit because he can't get the police union to stop paying the cops $1,000 to $1,500 bonuses every time they shoot somebody. The union position is the cop needs room, space, and time to decompress after such an experience and the bonus is so they can get out of town and take a little vacation. The mayor thinks it looks really, really bad.
I have never had to point a gun at anybody for any reason and God willing I will never need to do that. But I can't fault somebody who is experiencing severe bodily harm and does whatever is necessary to defend himself/herself.
Again none of us know the full circumstances and can only go by the conflicting information we're getting via the media. I wonder though, if those who are so convinced Zimmerman is the one in the wrong will accept it if the Grand Jury finds insufficient evidence to indict?