The problem really with the case, I think, was the police not taking the guy into custody initially. That sent out a bad message and got people's attention because it gave the impression that the authorities (for whatever reason) weren't taking the incident particularly seriously.
If the police had arrested him initially - even if they had then released him without charge - I doubt the case would have got half the attention it did.
And the difference US law (or is it just Florida?) compared to here, I think, is that here the burden would be on the defence to prove that the killing was in self-defence, rather than it being for the prosecution to prove it wasn't.