For all of us who are non-experts about how the Trayvon Martin case should have gone, the clearest explanation I have seen is on MSNBC's The Last Word.
If you go to about 4:20 a professor of Law and Police Studies at the John Jay School of Criminal Justice speaks. John Jay has a highly respected Criminal Justice Program. The professor, Eugene O'Donnell, was previously was a NYC cop and then a prosecutor, says it was pretty straightforward case. A case that new police sergeants across the country could have decided within 20-30 min after arriving on the scene would have decided an arrest should be made.
The issue isn't figuring out who killed Martin. It's absolutely known that it was George Zimmerman killed him, but the issue is if there was any justification for it. However, he says that is not for the cop on the scene to figure out. It's an issue for the jury.
With the 911 calls, there's a lot of evidence that was available very early in the case. And those tapes point to Zimmerman's state of mind right before the incident.
He notes that the an arrest is a preliminary judgement, not a final judgement. The final judgement comes before a jury when the defendant can present exculpatory evidence and has the presumption of innocence.
Basically the cops should have made an arrest within an hour.