Why would the first punch matter?
You are not allowed to kill people because they punched you. You are, with only a few exceptions, allowed to kill someone if you can convice a jury that you believed you were in danger of being killed or suffering a critical injury and that no other reasonable options were available to you.
The question in this case is not so much who hit who first, but rather does Zimmerman's decision to pursue Trayvon negate his right to employ deadly force in any subsequent physical altercation.
In my opinion, etc.
This guy pursued someone who was unarmed (unless you consider a bag of radios as 'armed'). This guy had a knife and stabbed and killed the guy with the bag. He was found innocent via SYG in florida.
http://www.miamiherald.com/2012/03/27/2717572/miami-dade-issues-ruling-in-stand.html