SANFORD, Fla. — From the moment George Zimmerman held up his arms and told the police that he had shot Trayvon Martin, one fact was undisputed: an unarmed black teenager lay dead.

But as one top Florida defense lawyer, Michael Band, said on Sunday, “Trials, for better or worse, are not morality plays.”

From the start, prosecutors faced a difficult case — weak on evidence and long on outrage. Mr. Zimmerman had the power of self-defense laws on his side, and was helped by a spotty police investigation and prosecutorial missteps. The initial investigation foundered when the local prosecutor balked at bringing charges, convinced that overcoming the self-defense claims would prove impossible.

But six weeks after the killing, his replacement, Angela B. Corey, from the Jacksonville area, charged Mr. Zimmerman with second-degree murder, a tall order.