This is the legal expertise of those on this thread.
At the scene: You, as the police, once he said he shot Martin, should have immediately told him: "You are under arrest for murder. You have a right to remain silent and anything you say WILL be used in court against you. You have a right to an attorney and if you cannot afford one, an attorney will be appointed to you."
Then at the station, you would again remind him you know he's a murderer, he's under arrest, and he gets lawyer, "we all know exactly what happened. Tell us exactly what happened as it exactly did so we can use that in court against you." Of course he's not going to say he wants to talk to that lawyer.
Then, in trial, you tell the jury to Defense questions: "Of course I immediately knew exactly what happened and that he's a murder. So, no, we didn't need any evidence to know that and no, we did not explore any other possibility before arresting him. And no, I didn't need to see any forensic testing results first. Why would we? Everyone knew immediately everything that happened. Besides, if I didn't arrest him immediately he would have fled the country. I know his type. And his type needs to go straight to jail. Grand juries are just a big waste of everyone's time.
No, other than Zimmerman, there were no eye witnesses to the shooting. But Zimmerman admitted he pulled the trigger. That's all we needed to know. That and that he called 911 that he was following Martin. That's murder, plain and simple, no exceptions, so there really was nothing else to investigate.
Instead, what we did is put together this collection of evidence against Zimmerman after we arrested and charged him to prove to you what we already knew for you jurors so all of you can all agree that I was right from the very start. Everything we put together all only shows that I was 100% right in my instant decision. No, believe me when I tell you that I didn't selectively pick evidence, i certainly wasn't out to get Zimmerman but completely fair and impartial towards him, believe me that there is no other evidence and no we didn't pointlessly look any other evidence - because I knew in the first 15 minutes exactly what happened.
Besides, that new forensic stuff isn't necessary. Law enforcement did just fine without it for over 200 years. Why fix what isn't broken? We don't have time for that anyway, at least not when a case is immediately solved.
Trust me, I'm in law enforcement, I know what I'm doing and anyone I say is guilty always is. I'm very proud of my perfect conviction record and my time efficiency in solving crimes. I don't wasting my time. Its actually quite easy for me to know who's guilty and who's not. In fact, 100% of everyone I say is guilty had been found guilty."
THAT is what most on this thread are furious didn't happen. In your minds, the trial certainly should last more than 30 minutes, an hour tops.