I think there needs to be a standard for being in fear in order for you to kill someone.
I think the standard is met after your door is broken down.
I think the standard is NOT met by a knock on the door (and I live at the end of a dirt road in the country, a knock on my door after dark would be very scary).
In between the knock on the door and the perp in your house, there are a million shades of gray, and I would want the DA to inquire after the facts if someone I cared about was dead.