That's only true because of the very unusual Stand Your Ground law in Florida. In almost any other place on earth self-defense has to be a reasonable response to the aggression. If you punch me in the arm, I can't shoot you and claim it was self-defense. Or, as is relevant in this case, if you push me to the ground, I can't shoot you and claim it was self-defense (unless, maybe, you continued to attack me on the ground and were physically much larger than me). If you point a gun at me, then I can shoot you, though I still have an obligation to avoid killing you if I can.