Have you heard of the castle doctrine [1]? It's resulted in some really crazy (and imho horrible) situations where private citizens are allowed to use deadly force and literally murder people in cold blood.
This guy wasn't even on his porch. Stand your ground laws essentially allow gun owners to claim they were frightened for their lives so that becomes "their castle". Wouldn't be very hard to repurpose this kind of weaponized law to actually defend your household.
btw - castle doctrine was intended to apply within property lines. If someone is on your doorstep, they are likely already clearly on your property.
All of this is theoretical but honestly I wouldn't be surprised if this is used for exactly what you said it shouldn't allow.
According to the zimmerman trial, if you look into it at all, Zimmerman claimed that he was being attacked, got knocked over on the ground, and then while the attacker was on top of him, yelling that he was going to kill him, he defended himself with a gun, at that point.
And there was some physical evidence and injuries supporting this narrative.
You can say that his statement is a lie, or that you don't believe his testimony.
But, if that testimony is true, Shooting someone who is on top of you, assaulting you, yelling that you are going to die, while you are on the ground, is a much different situation than shooting someone who just knocked on your door.
I would really recommend you look up that case more, if you thought it was in any way similar to "just shooting someone, at your door".
Whatever. Coming into someones house to make threats is very stupid idea.
One day I came into my kitchen, there is my wife crying, and three big threating guys I do not know. I ask them to leave, instead of heading to door, they are aggressive and surround me. What do you thing is going to happen?