Extended Response to "Zimmerman's Murder Charge: Prosection Must Prove 'Depraved Mind'

In late February, a boy named Trayvon Martin was shot and killed by a man named George Zimmerman. There has been some debate about whether Zimmerman really only killed Martin out of self defense or it was simply murder. Zimmerman was recently taken into custody without any chance of bail. I personally think that   Zimmerman is guilty and had no right to pursue and kill Martin, but do you think about all of this? Was Zimmerman acting in self-defense, or was it a deliberate kill. Let's see both sides of the story:
Prosecution: George Zimmerman was patrolling the "gated" community in Sanford at night when he saw Trayvon Martin walking alone with a hood over his head. Zimmerman thought that he looked suspicious, and began to follow him. As he was following him, Zimmerman called the police. The police ordered Zimmerman to stop following Martin, but he ignored them. This is the part that I don't understand: if the police told Zimmerman to stop following Martin, why did he continue to pursue him? Later, when Zimmerman caught up with Martin, there was most likely interaction and perhaps a struggle: "The 17 year-old was unarmed. Zimmerman told police he acted in self-defense" (Zimmerman's Murder Charge: Prosecution Must Prove 'Depraved Mind'). If Trayvon Martin was unarmed, and Zimmerman had a gun, how could it be self-defense?
Defense: The other opinion is that Trayvon Martin put up a fight and maybe even tried to injure Zimmerman when Zimmerman didn't even do anything yet. It is possible that Martin thought he was being mugged, and wanted to defend himself. If that is true, then it would all be a misunderstanding that went wrong. Despite this point, Zimmerman was charged with second degree murder.
Whatever really happened, George Zimmerman screwed everything up by ignoring the police's order and continuing to follow Trayvon Martin when it was unnecessary. Some people think that Zimmerman only thought Martin was suspicious because he was African American. From hearing both sides of the story, I am confident that most people would say George Zimmerman is guilty of murder.