My own personal feeling is that GZ had every legal right to follow Trayvon, however I am not sure what the legal ramifications are of him approaching TM and questioning him. Especially since we will never know the actual details of that encounter. I do question how it happened that in the process of questioning someone else, a fight ensued.
I again would have been yelling my fool head off.
GZ had a right to watch TM, but did he really have cause to call the police? GZ did not witness TM doing anything a normal person would have seen as suspicious. By calling the police, GZ escalated the situation.
When GZ got out of his truck to run after TM, he escalated the situation once again. GZ's comment "these ******** always get away" is a clear sign that GZ believed TM was a criminal.
We know that a confrontation happened between GZ and TM and it quickly escalated into a fight. We don't know for certain that GZ started the fight, but it's in his character to do so. He has already escalated the situation twice in a matter of minutes.
About a minute later, GZ escalated the fight into a homicide. TM wasn't armed, he wasn't prepared for kill someone. He was just walking back from the store after buying some snacks. GZ was the one who brought a loaded weapon.