In Florida, the law reads this way: "a person is justified in using deadly force (and does not have a duty to retreat) if he or she reasonably believes that such force is necessary to prevent the imminent commission of a forcible felony, or to prevent imminent death or great bodily harm to himself or herself or another." [Florida Statute Section 776.012.]
"The use of deadly force is further justified when a person is resisting any attempt to murder such person or to commit any felony upon him or her or upon or in any dwelling house in which the person is located." [Florida Statutes Section 782.02]
In Washington, if someone claims a killing was done in self defense, then the jury would be instructed this way: " It is a defense to a charge of murder that the homicide was justifiable as defined in this instruction. Homicide is justifiable when committed in the lawful defense of the slayer when:
1) the slayer reasonably believed that the person slain intended to commit a felony or to inflict death or great personal injury;
2) the slayer reasonably believed that there was imminent danger of such harm being accomplished; and
3) the slayer employed such force and means as a reasonably prudent person would use under the same or similar conditions as they reasonably appeared to the slayer, taking into consideration all the facts and circumstances as they appeared to him, at the time of and prior to the incident.
The State has the burden of proving beyond a reasonable doubt that the homicide was not justifiable. If you find that the State has not proved the absence of this defense beyond a reasonable doubt, it will be your duty to return a verdict of not guilty." [Washington Pattern Instruction #16.02]
The jury would also be instructed: "It is lawful for a person who is in a place where that person has a right to be and who has reasonable grounds for believing that he is being attacked to stand his ground and defend against such attack by the use of lawful force. The law does not impose a duty to retreat." [WPI # 16.08] For many years this 'no duty to retreat' instruction was derisively referred to as 'the law of the Old West.' Washington was one of many Western states that used this instruction.
The key ingredient in both Florida and Washington is the "reasonable belief." To use the facts of the Trayvon Martin case, the state will have to convince the jury that Mr. Zimmerman's use of deadly force was not based on a reasonable belief. Put another way, first, was he reasonable in following Martin after the 911 operator told him not to? And more importantly, did Zimmerman reasonably believe that Trayvon Martin was about to kill him or commit a 'forcible felony' or cause great bodily harm? Suppose that Zimmerman testifies that Martin punched him in the face or pushed him away, or even kicked him. Is that sufficient for Zimmerman to reasonably believe Martin would kill him or cause great bodily harm? Preventing a bloody nose or a black eye do not seem to be sufficiently threatening so as to justify deadly force, i.e., pulling out a gun and shooting.
I have had several murder trials where deadly force was used in a situation where some force would have been justifiable. In one case, my client agreed to leave the bar and face a bully who was tormenting him. They began a fistfight. Then during the tussle, my client pulled a steak knife from his sock and began stabbing his opponent until the fight broke up. The other man died of his wounds two weeks later. Two juries heard this case (1st one reversed on appeal) and both convicted.
Another case involved a client sleeping in his room. His mother's boyfriend, a drunk and a bully, suddenly burst into the sleeper's room and began a fight. My client pulled out a large kitchen knife he kept in his room and cut him more than a dozen times, killing him. Some of the wounds were 'defensive'--to the hand and fingers, so a murder charge was filed. The first jury voted 7-5 to acquit but could not reach a decision. The re-trial jury convicted him of the lesser included offense of manslaughter. And a third case involved a client who got into a fight near his home with his sister's ex-boyfriend. When the ex-boyfriend began threatening to kill my client, he ran into his house got a .45 caliber gun and shot the ex-boyfriend from the front porch about 6-8 times. The jury convicted him but said if he had only fired one time, they might have acquitted.
The similarity in the three cases is that the jury probably found that the force used was greater than necessary. Washington law adds the requirement that the force be what a "reasonably prudent person" would use, but arguably the term "reasonable belief" in Florida can encompass that.
The biggest difference may be the jury. On the same facts, juries will differ in deciding what is "reasonable." Stereotypically, in the South, or in rural areas, there is more tolerance for excessive force in self defense, but not always. Another major factor in the Florida case will be Zimmerman's credibility and how he does on cross examination. Since there are no other eyewitnesses, his version will largely be uncontested--except for the 911 calls. A well prepared defendant will be advised to avoid contradicting those calls.
In the end, the law that applies in Florida will not be that different from what is applied here in Washington. Since the details of Zimmerman's story is not publicly known, there is no way to predict what the jury will do. Whatever they do, it'll hit the fan hard.