The details of the case, now well-known around the country, illustrate just how muddy "stand your ground" laws can become. Twenty-five states, including Georgia and Florida, have virtually identical laws. Stand your ground laws provide immunity from prosecution if you kill someone while defending yourself.
In Georgia there have been 21 cases of justifiable homicide since 2003, according to the Georgia Bureau of Investigation. Those are homicides in which the police at the scene, prosecutors later on or a judge in an initial hearing decided no crime had been committed.
Many prosecutors, defense attorneys and gun rights advocates agree there is a need for the laws that allow deadly force in some circumstances, but there are some who disagree.
“We were adamantly opposed to the shoot-first-ask-questions-later law,” said Brian Malte with the Brady Campaign to Prevent Gun Violence. “That’s a recipe for tragedy.”
Still, the courts have said for more than a century it's legal to "stand your ground" with deadly force when faced with a threat of death or serious injury.
Georgia's Supreme Court wrote in 1898 -- and many times since -- there is no requirement that a victim of an attack first try to escape before using deadly force to stop an aggressor. The U.S. Supreme Court ruled also in 1898 that "a true man does not fly in the face of an aggressor who seeks to do him grievous bodily harm."
"That came to be known as the ‘true man rule' and that has evolved into the stand your ground rule," said University of Georgia law professor Ron Carlson.
It has only been in recent years that states have included those court decisions in their laws. Florida's adopted a stand your ground law in 2005 and Georgia in 2006, expanded upon the self-defense rights the courts have upheld for years. There are 23 other states with similar laws that allow for justifiable homicide with no requirement that the victim first try to escape; seven other states allow residents to defend themselves with deadly force in specific areas -- such as at home in the car or at the workplace.
Self-defense claims are made often in homicide cases, Georgia prosecutors say. But it's hard to make a legitimate claim of self-defense.
“Rational, reasonable citizens who use deadly force to protect themselves... are fairly rare," said Gwinnett County District Attorney Danny Porter.
In Dekalb County, said chief assistant district attorney Don Geary, "In half the drug murders we get, they claim it was self defense.... The problem with self-defense is if you're in the commission of a felony, you can't claim self-defense."
A key question in the Martin shooting is whether Zimmerman was the aggressor. If so, Carlson said, he was not justified to use deadly force.
"Can you say ... ‘I triggered this thing...but then I backed off ?'" Carlson said. "When do you stop being the aggressor?"
But Martin’s family deny teenager attacked Zimmerman. They believe he was an innocent victim.
Often there is only one witness, the person who pulled the trigger or had the knife, and that person is claiming self-defense.
"Sometimes you ... make a judgment call based on all the evidence [and] whether the acts were reasonable," said Gwinnett prosecutor Porter. "The standard is whether or not a reasonable man would have acted the same way under the circumstances."
And even if police and prosecutors believe there was no self-defense, a jury can find otherwise.
Lona Scott shot her husband, Cliff, six times during an argument in their bedroom on March 4, 2008, after the 42-year-old trucking executive had transferred assets totaling $5 million into a bank account in the Bahamas and was divorcing her.
The police and prosecutors believed it was murder. Lona Scott said it was self defense. She was indicted 14 months later.
The Dunwoody mother of two argued at trial she had no choice, she couldn't escape so she had to stand her ground and kill her husband before he killed her.
On Feb. 2, 2010, a jury deadlocked 11-1 in favor of guilty. When she went on trial a second time, she was acquitted.
"If you're truly a victim, you don't have to wait and give the man or the woman (attacker) another chance," said Lona Scott's defense attorney, Brian Steel.
Cliff Scott's family was stunned by the verdict.
Josephine Scott said a "stand your ground" claim is appropriate sometimes but it was not in the death of her son.
"How many shots constitute self-defense?" said Josephine Scott. "The first shot severed his aorta. The next one pierced his elbow... There were two [shots] to the elbow. The fourth shot was between the eyes. Then she shot him twice in the back as he was flat on the ground. You call that self-defense?"
And sometimes it's easy to decide deadly force was justified.
Law enforcement officials said there was little question Georgia's stand your ground law applied to Georgia Tech student Lewis Moore when he shot and killed 30-year-old Yuhanna Abdulah Williams in December 2010.
Moore was getting out of his car at a Conyers grocery store when Moore grabbed him, put a knife to his throat and demand money and car keys.
Moore told police he grabbed his Taurus .357 Magnum from its a holster, turned and shot Williams in the face.
The first thing he said to a Rockdale County deputy was "this guy tried to rob me and I shot him."
"It was a clear-cut case of self defense," said Rockdale County Sheriff's Office investigator Michael Camp.
Moore was not charged.