I think those are valid questions. It is perfectly plausible that a 17-year-old might think he needs to take action. Which is probably why the 911 operator told Zimmerman not to follow him. (If there was a law that you must obey what emergency operators tell you, I might be able to live with that BTW).
That was an extremely poor, and tragic choice by Zimmerman - but not a crime. Zimmerman is going to have to live with whatever the truth is. WIth what we know, though, I don't see any way to convict him (Zimmerman) of a crime. We want justice, but what exactly is that in this case?