There is a very old truism for the law that in controversies of life and death, the only real resolution can come by a fair trial. It was as true in the Old West as it is today.
The only way George Zimmerman will ever be truly free is to be found not guilty by a jury. There is no verdict of “innocent”, but that label may be applied by a court or a prosecutor if evidence comes to light that completely exonerates a person of the indictable offense.
That is, “They couldn’t have done it because they have an airtight alibi at a place far away from the scene of the crime.”
And “free” is relative. Like O.J. Simpson, he might be found not guilty, yet the Martin family may sue him for wrongful death, for which he could get a large fine assigned him.
While I think it is pretty unlikely, if Zimmerman does succeed in the "stand your ground" hearing, he becomes immune from civil suits over Trayvon's death.