He should not have gotten out of his truck. He should have observed and called 911 and not gotten involved. I wonder if the national sheriff association approved his rescue technique.
Honestly, I'd be more sympathetic to Zimmerman's judgement calls if he hadn't called 911 first; then there'd be no question of whether he should have pursued after being told he didnt' need to.
The two events are not comparable.
In the case of the traffic accident, you have an immediately apparent need (person trapped in car) with a possible element of time sensitivity depending on the nature of the crash. Zimmerman saw the need and offered a solution. The story had a happy ending.
In the case of a person seen in the neighborhood that Zimmerman determined was suspicious looking, you have an ambiguous situation involving a person who may or may not be up to no good and may or may not be armed (in the end, he wasn't, but Zimmerman did not know that). It concerned him enough to call 911 but was ambiguous enough that he didn't immediately take direct action. Zimmerman at some point after the call judged it necessary to pursue, which ended in a physical confrontation and young man dead as a result. The story had a sad ending.
When stories have a happy ending, the good samaritan is praised, God is thanked, and if the rescuer is of a different race from that of the victim, interracial cooperation and harmony are heralded.
When the story has a sad ending, people want to place blame. Who is responsible? Justice must be done! Lines are drawn, sides are taken, and people demand restitution. Since the sad ending brings out negative emotions, this often happens in a way in which accuracy takes a back seat to a need to win and in which the sense of loss at the death of another human being is diminished in favor of one side gaining victory over the other.
Demonstrators who are angry about the verdict are not honoring Martin. Honestly, I don't think they care one whit about him. They're simply mad that a trial in which they had no part didn't go the way that they wanted it to.
People who are supportive of the verdict and of Zimmerman's actions aren't bothered by the loss of life; they are more concerned with how the trial's verdict might impact some part of their own life, be it self defense, the right to carry a gun, or the idea that a man may be convicted in order to pacify the crowd that threatens to riot if things don't go their way.
In the end, after the confrontation with Martin, a seventeen year old boy was dead and his family was grieved. Every flaw he had and bad decision he ever made is now laid bare for the world to see, and regardless of what verdict the jury had reached, that young man is dead.
The death of another human being diminishes us all in some way.
Also, whenever a person is killed by another, there are consequences. They aren't always the same; a police officer killing a man who draws a gun on him will have different consequences from that of a man pulling a gun on and killing a police officer. Which are different from those of a young child who kills a sibling or themselves while playing with the gun they found in their parent's bedroom. But when life is taken, there are always consequences.