784.011 Assault.—
(1) An “assault” is an intentional, unlawful threat by word or act to do violence to the person of another, coupled with an apparent ability to do so, and doing some act which creates a well-founded fear in such other person that such violence is imminent.
According to what I’ve read, the dead guy didn’t threaten by word or act, he just did the deed. That’s Battery according to Florida law (and that of other places):
784.03 Battery; felony battery.—
(1)(a) The offense of battery occurs when a person:
1. Actually and intentionally touches or strikes another person against the will of the other; or
2. Intentionally causes bodily harm to another person.
Where does that leave us? Well, according to what I’ve read, the shooter was “talking” to the dead guy’s woman and doing so loudly enough to attract the attention of passersby. We don’t seem to know whether or not his talk constituted an “unlawful threat by word or act to do violence to” the woman, but if it did his talk constituted an assault. The dead guy may have reasonably believed the shooter was assaulting his woman (threatening by word or act to do violence) and acted in accord with the Florida Stand Your Ground law which states:
776.012 Use or threatened use of force in defense of person.—
(1) A person is justified in using or threatening to use force, except deadly force, against another when and to the extent that the person reasonably believes that such conduct is necessary to defend himself or herself or another against the other’s imminent use of unlawful force. A person who uses or threatens to use force in accordance with this subsection does not have a duty to retreat before using or threatening to use such force. (Emphasis added.)
I wrote “the dead guy may have believed”, but we don’t know what he believed. I believe the whole thing needs to be put before a jury.
Maybe, if one has to put someone else on the ground, it’s advisable to close with the adversary and make sure they aren’t able to harm you when you retreat, just by disarming them if possible and necessary. That’s a hard problem.
That's not the Stand Your Ground law, it's self defense. SYG is only that portion that says, "A person who uses or threatens to use force in accordance with this subsection does not have a duty to retreat before using or threatening to use such force."