“Self defense in your own home or business was always legal and the new provisions of SYG aren’t required in those cases.”
You’re right, SYG doesn’t apply. Within the home it’s called the “Castle Doctrine,” as in “an Englishman’s home is his castle.” No, it is not in all states and all instances assumed you have the right to protect your life and property against invaders. For instance, I remember a firestorm some time back against a man who rigged a shotgun to fire when the door to his shop was breached. Some potential robber got his head blown off, and people were angry at the owner instead of happy that another criminal got what he deserved.
It was kind of a general statement with focus on Florida, where I'm sure self defense is generally legal in your home or business.
Your example is not a very good one to demonstrate your first statement, however, as many jurisdictions differentiate between self defense per se, and placing a "booby trap". Booby trapping is illegal in many jurisdictions in which self defense is perfectly legal and expected, and in many cases is justified by the risk to LEO's and others who might claim some sort of legal right to enter your property. Personally, I'm not entirely swayed by that reasoning, as the circumstances under which they have good reason to enter without permission and without prior notice would seem to be vastly outnumbered by the cases in which booby trapping, when rationally applied, would lead to a beneficial outcome, as in your example.