@Thomas,
Just because "we don't know" who started the fight that night doesn't mean we can't judge Zimmerman's other actions before, during and after. When his actions lead to the death of a person and those actions can be considered reckless then he can be found guilty of a crime.
Would a reasonable person have stayed in the car?
Would a reasonable person have followed someone in the dark?
Would a reasonable person have let the police handle it?
Would a reasonable person have continued to follow after being told by a 911 operator "we don't need you to do that?"
At what point does a series of known "unreasonable" acts by Zimmerman suddenly overshadow what we don't know?
In a local case a father was just convicted of child endangerment and manslaughter because his 4 year old found a gun and shot and killed himself. There are a lot of things we don't know in that case but the recklessness of leaving a gun where a child can get it overshadowed what we don't know.