We’ve discussed all this before. Many people can be legitimately described as murderers even if they weren’t ultimately convicted (for example – KKK murderers who weren’t prosecuted or who were “exonerated” by a white supremacist judicial system in the early and mid 20th US). “Murderer” is not only a legal term, it’s also used outside of purely legal contexts.
As for the specifics of the Zimmerman case, that’s been done to death in other threads (and would be a hijack here), and the two of our systems of morality have been demonstrated to be so profoundly different that there’s no possibility of any better understanding here on what we find morally reprehensible.