While there may have been mitigating factors - Zimmerman's state of mind, and his unreasonable fear during the scuffle - I doubt the prosecution could legally bring any charge other than unlawful homicide, more commonly known as murder. The US legal system is based upon English Common Law, and the elements which comprise murder and manslaughter would essentially be the same.
In New South Wales Law, which is identical in most important respects to British Law, unlawful homicide is divided into four categories -
Murder
Constructive Murder (or Felony Murder)
Voluntary Manslaughter (analogous to 2nd Degree Murder)
Involuntary Manslaughter
For murder (except in the case of “constructive murder”), the prosecution must prove that the accused killed with:
* an intention to kill; or
* an intention to inflict grievous bodily harm, being bodily harm of a really serious kind; or
* a reckless indifference to human life, meaning that the accused foresaw the probability that death would result from his or her act or failure to act.
For voluntary manslaughter, the prosecution is required to prove the same mental state as is generally required for murder. That is, the accused must be shown to have either intended to kill or to cause grievous bodily harm, or to have been recklessly indifferent to human life. However, the accused will be convicted of voluntary manslaughter instead of murder where his or her mental state was affected in a way which is recognised by law to reduce his or her culpability for the killing. The factors so affecting the accused’s mental state must be shown to constitute either provocation, diminished responsibility, or infanticide. These are known as the partial defences to murder, and require proof of some form of mental impairment or loss of self-control which significantly affected the accused’s culpability at the time of the killing.
Involuntary manslaughter is an unlawful killing by a person who cannot be proven to have the requisite guilty mind for murder, but whose conduct falls short of conduct expected of a reasonable person in the same circumstances. An accused may be convicted of involuntary manslaughter by an unlawful and dangerous act or by criminal negligence. Manslaughter by an unlawful and dangerous act requires the prosecution to prove that death was caused by a sufficient kind of unlawful act and that a reasonable person, engaged in the same conduct as the accused, would have realised that he or she was exposing another to an appreciable risk of serious injury. Manslaughter by criminal negligence requires the prosecution to prove that the accused’s act or omission causing death involved such a great departure from the standard of care to be expected from a reasonable and prudent person as to deserve to be called a crime against the community generally and conduct deserving punishment.
Lawlink NSW: 2. The Murder/Manslaughter Distinction in Unlawful Homicide
Felony murder deals with killings occurring during the commission of other crimes (such as bank robbing), and does not concern this case.
It cannot be reasonably claimed that Zimmerman, at the time of shooting Martin, did not intend death or grievous bodily harm. His intent was obviously malicious at that moment, but what makes his crime different from murder, is that he did not set out at any time prior to the scuffle, to kill the 17 year old. It was not therefore a planned homicide, and there was no obvious malice aforethought.
The circumstances indicate however, that it was not an accidental shooting, nor was it merely the result of criminal negligence on Zimmerman's part (as might be the case with leaving a loaded firearm where it might be misused, or the case with reckless driving causing the death of another motorist, or a pedestrian.) So involuntary manslaughter (simple manslaughter in US terms) cannot be held to apply.
So, without setting out with the intent to kill, but killing with the intention of doing so at the moment, does not allow a charge of murder in the first degree, and it does not allow a charge of involuntary manslaughter (manslaughter in the US,) so it only really only leaves voluntary manslaughter - or murder in the 2nd degree - as the logical charge to be brought.