You have to remember that the police and districts attorneys in Florida have to deal with the stand you ground law. That law say that if you are in a place you can legal be in you may use force, including deadly force to prevent death or great bodily harm, without the need to retreat.
Actually, not quite. I posted FL's Castle Doctrine law on the last page. It requires the victim to actually have been on the shooter's property, among other things.
Also, as an affirmative defense, the defendant has the burden of proof to show the defense is valid. The state still has to prove all the elements of the crime (probably manslaughter here), and the defendant would have to show the Castle Doctrine applied. (IMO, it doesn't in this situation.) There is no reason the State couldn't file charges and start the process. Maybe it turns out that the shooter does have a Castle Doctrine defense. If so, he would be able to put on evidence of it (and remember, the defendant only has to create reasonable doubt) and maybe get acquitted.
Plus the law also says that if the state does bring charges and the person is found innocent because of this law they are owed the cost of their defense.
Not quite. The defendant isn't liable for "costs or fees of the court or any ministerial office, or for any charge of subsistence while detained in custody." They are liable for attorney's fees, and the law does not say the state has to pay the defendant fees. The fees are just waived.