Listening to the recordings, makes it appear less like a racially motivated hate crime. Zimmerman did not seem to notice the victims colour as much as his clothing. He was concerned that he was hiding a weapon because he had his hands in the front pouch of his hoodie. His 911 call description is that of someone who honestly just seems to be reporting a suspicious person. What bugs me is that he notices the victim looking around, and then the victim starts to run, at which point he pursues. He actively runs after the victim as can be heard by the sound of the wind on the microphone in the phone. However, before the attack actually happens he has stopped running and hangs up the phone.
Now, in the other recording you can hear someone screaming quite loudly in the background, and that is screaming right up until the gunshot sound. Based on the voice of the suspect, I would be inclined to say it was more the victim screaming. However, I don't think it is solid enough as my assessment on the voice is far from good. I am aware that peoples voices sound very different when screaming. The voice in my opinion though sounds of a higher pitch than Zimmermans voice.
As for the suspicious behavior of the victim, it makes sense. He was visiting his father with this brother. They were not where they normally go, as they were at his fathers girlfriends house. Given it was dark and raining it is quite likely that he was a little lost in the complex area, and looking at the houses to find the correct house number. Running away from zimmerman also makes sense because at that point zimmerman is watching him while on the phone and looks pretty suspicious too.
I think I would move towards manslaughter instead of murder. However as has been stated, Florida's messed up laws mean that this guy will walk and open a whole slew op opportunities for this law to be abused.