HOMICIDE

Word homicide means:

The deliberate and unlawful killing of one person by another; murder.
"Homicide is when a man of sound memory, and of the age of discretion, unlawfully killeth within any country of the realm any reasonable creature in rerum natura under the King's peace, with malice aforethought, either expressed by the party or implied by law, so as the party wounded, or hurt, etc. die of the wound or hurt, etc. within a year and a day after the same."
Although the term homicide is sometimes used synonymously with murder, homicide is broader in scope than murder. Murder is a form of criminal homicide; other forms of homicide might not constitute criminal acts. These homicides are regarded as justified or excusable. For example, individuals may, in a necessary act of Self-Defense, kill a person who threatens them with death or serious injury, or they may be commanded or authorized by law to kill a person who is a member of an enemy force or who has committed a serious crime

Types of Homicide

Justifiable 

 Justifiable homicide is "a killing without evil or criminal intent, for which there can be no blame, such as self-defense to protect oneself or to protect another, or the shooting by a law enforcement officer in fulfilling his/her duties."

This might apply if Martin attacked Zimmerman without provocation. It seems unlikely to me that this would be the right perspective on the crime since Zimmerman was following Martin, against the advice of police, and clearly drew Martin's attention. 

Excusable Homicide

 Excusable homicide is "The killing of a human being, when the party killing is not altogether free from blame, but the necessity which renders it [is] excusable, may be said to be partly induce[d] by his own act."

To me this looks like we're getting closer to an accurate viewpoint. It reads like it would apply if Zimmerman started the confrontation, things got heated, and he tried to withdraw, but Martin would not allow it. In that case, if Zimmerman killed Martin it might be excusable homicide.

The devil, of course, is in the details.

According to Corey's statements both justifiable homicide and excusable homicide can be an "affirmative defense" for Zimmerman if the situation warrants it. For that to be true, however, Martin would have had to be the aggressor, or pursued Zimmerman if the shooter tried to flee.

The former seems impossible to me since Zimmerman was initially pursuing Martin. The second seems unlikely, if only by applying Occam's Razor: How hard would it have been for Zimmerman to use a gun to persuade Martin to stop pursuing him?

My guess, based on these definitions and Corey's statements, is that both options were examined and discarded before charges were filed. Neither justifiable nor excusable homicide seems to apply, in my amateur opinion. 

Murder

Murder is typically broken down further into two categories:

    First-Degree Murder

    Second Degree Murder

First-Degree Murder: 

First-degree murder, most serious of all homicide charges, is reserved for situations where the person accused of the homicide planned the killing and intended for the victim to die because of his or her actions. Famous cases involving first-degree murder include the Charles Manson, O.J. Simpson, and Scott Petersen cases out of California. These cases are usually the most heinous crimes and as such, the most severe punishments are usually reserved for these convictions. In many states, those convicted of first-degree murder will face the death penalty or life in prison.

Second-Degree Murder: 

Second-degree murder is a charge reserved for cases where it cannot be proved that the killer planned the killing but where the killer still intended that the victim die because of her or her actions. A recent famous case involving a second-degree murder charge is the Phil Spector case out of California. Situations where second-degree murder charges arise are sometimes described as "crimes of passion" or instances where the killing occurred in the "heat of passion." In some states, individuals convicted of this crime can face life in prison with or without the possibility of parole, depending on the state’s laws and the situation’s circumstances.

Manslaughter

The charge of manslaughter is reserved for instances where the accused did not plan the crime nor did he or she intend for the victim to die because of his or her actions. Manslaughter charges usually arise out of accidental circumstances where a person died because of the event. A manslaughter case in recent history is that of Michael Jackson’s doctor, Conrad Murray. Sentences for those convicted of manslaughter vary widely depending on state laws and the circumstances of the event in question.