'Murica, guns, and laws - Oh My!

Yes, I'm a Brit! Yes, I am interested in how things play out in America in respect of heir gun laws.
No, I'm not like Piers Morgan preaching for gun control.
Ok, now that's out of the way I can get on with things.
A lot has happened in the USA recently by way of prominent crimes and especially interms of Gun Crime, and what people want to see done to prevent more tragic happenings.
Today I'm looking at two of these instances; George Zimmerman and Marissa Alexander.
In case you missed it, George Zimmerman has been acquited of any wrong doing in the death of Trayvon Martin, and so walks away a free man.
Marissa Alexander fired warning shots at her abusive husband and is sentenced for 20 years.
I'm only talking about these cases now because it's only recently that he respective juries have passed their verdicts. My own opinions as toi the guilt of the parties involved is no irrelevent, as is everyone else's - the descision has been made (barring the appeals process).
So what I'm trying to look at here, is how one person can kill another and walk away almost untouched by it, while someone else spares a life by giving fair warning ends up in jail for 20 years. That to me seems utterly crazy.
Maybe both cases are from different states, so might have different laws affecting them? No. Both cases are in Florida.
Throughout the Zimmerman case people have been crying foul on race reasons - but I can't see how that's valid as Zimmerman is hispanic, and the race reasons cited are white supremacy. I'll classify that one as crazy talk.
Marissa is black, and female - it's possible that both her gender and her race counted against her. Possible, but based on the (lack of) media coverage given to the case it's impossible to judge.
My best guess - Zimmerman had a better lawyer behind him, and a more lenient Judge. Why do I say that? Well, Zimmerman waived his right to a 'Stand Your Ground' pretrial immunity hearing, when Marissa Alexander's lawyer made an appeal under the 'Stand Your Ground' law, the judge denied it.
the 'Stand Your Ground' law, according to the wikipedia, says:
"In the United States, stand-your-ground law states that a person may justifiably use force in self-defense when there is reasonable belief of an unlawful threat, without an obligation to retreat first."
There's a lot more to it than that, if you want to read the whole thing it's here:
http://en.wikipedia.org/wiki/Stand-your-ground_law
So, in short, ifyou feel you're in evere danger of harm or worse, you can 'stand your ground' and use force. To my uneducated eyes it seems that Marissa's mistake was to fore warning shots. The second she did that instead of shooting at her abusive husband, she lost the 'Stand Your Ground' backing because a warning shot shows premeditation rather than the instinctive self defence action of shooting her assailant.
In a country where a woman was brutally beaten, in front of her child, by a burglar, the fact that in a siilar situation an armed woman who fired a warning shot could end up with more jail time than the guy burgling her confuses and scares the heck out of me.
I would love to think that if a firearm was used in self defence or home defence you wouldn't face such stiff, if any, penalties.
I envy a country where people can legally use a weapon to defend their home, but the way the country implements its laws, and threatens their very existance in a highly violent modern age worries me.