There is not evidence that Martin was engaged in criminal activity when Zimmerman reported his presence to police. So your “save the taxpayers money” argument is hockey pucks. Florida is a death penalty state but does not levy the death penalty for assault (if Zimmerman’s account is 100% factual) nor does it levy it for having been suspended for suspected marijuana possession.
You apparently do not live an area where there is a neighborhood watch ( or a need for one). Zimmerman did EXACTLY the right thing. You no doubt live somewhere that the crime level is close to zero. Neighborhood watches have been set up precisely to have people in the area paying attention to those in the neighborhood. Calls to the police for this purpose are not unusual.
Zimmerman has the right of self defense. I wonder if your nose had been broken and a young male was on top of you pounding your head into the cement multiple times....would you shot the attacker? or would you wait to see if you would die from your head wounds?