You've heard wrong. The law is actually very much on Zimmerman's side, and the evidence leans somewhat towards him as well. Plus "having it out for Martin" is not something to convict him on. This is a big reason why we why we have a legal system: to protect he accused from being judged in the court of public opinion rather than on the true merits of the case.
The problem is his lawyer is grossly incompetent. Who opens a murder trial with a joke? A knock-knock joke, even. Another thing is that the the jury may very well feel that they have to do "something" due to the high profile of the case and convict Zimmerman of manslaughter, regardless of whether the state really proves its case.
From what I've seen Zimmerman seems like a douche who nevertheless is not guilty of murder. But we'll have to see how the trial goes.
And lastly, I find it so bizarre that this case is getting so much attention. Murder trials happen every day. Murder trials involving young black men as victims happen every day. There's no shortage of cases involving a racial angle, so why this one? It seems strange that this case, while devastatingly tragic to the people involved, has captured the country's imagination and not, say, this one where a group of black Marines robbed, raped, and murdered a white Marine and his black wife. That sure sounds like a trial of the century to me, not a tragic but also tragically run-of-the-mill shooting.
http://losangeles.cbslocal.com/2013/06/05/3-marines-found-guilty-of-murdering-couple-during-home-invasion-robbery/