Interestingly, even if you accept GZ's statement as the absolute truth, zero errors, he is still guilty by his own admission of at least manslaughter if not murder two. By his own admission he stalked this kid, at night, in the rain, even after the kid attempted to run away. The actions he admitted to would cause ANY reasonable person to fear for their life.
And with that in mind, even if Trayvon attempted to take him out first, Zimmerman loses any legal right to use lethal force. If you make someone believe that you intend to hurt or kill them, which is exactly what Zimmerman did, you don't get to kill them when they retaliate.
IN MY OPINION.
None of this had to happen. Zimmerman could have done what any reasonable concerned citizen would have done, he could have left it in police hands OR politely approached the boy in a non-threatening non-confrontational manner, keeping his distance, perhaps saying something like, "Excuse me, I am sorry to bother you, but I am the neighborhood watch and I noticed you were out in the rain. Do you need some help? Are you locked out? Do you live around here? Is there anyone I can call for you? No? You live right over there... that's great! Welcome to the neighborhood! My name is George and if you ever need any help don't hesitate to call me!"