Zimmerman was also the neighborhood watch captain - it's basically his duty to keep an eye on anyone who's acting slightly suspicious. Also, the dispatcher's line about telling Zimmerman that he didn't need to keep an eye on Martin was a canned line and not binding in the slightest. If you were about to get shot by another person in front of me and I called 911 to tell them about it and they said "Well the closest police cruiser is 5 minutes away" and I said "Well, this guy is gonna shoot my friend in 30 seconds, I'm gonna go stop him" they will actually respond "You don't need to do that" - it's nonbinding and non-authoritative and ignoring such instructions is perfectly legal.
Here's a video I might suggest some people take some time and watch. Now, I have no doubt that this guy is at least somewhat biased (mostly because of his little pot-shot at Obama towards the end), but he presents a lot better case than I've seen anybody else present. If someone wants to check his sources and other reputable, non-media sources and rebuke this guy's arguments, please feel free - in fact, I eagerly await your rebuttal.
https://www.youtube....Ax5E8EJc#at=442