Some states go by the common law which says basically that if someone breaks into or forces his way into your home you may presume that he is a danger to you and/or your family. The basic concept is that you can presume he is armed since no one forcibly enters a home without being armed.
Unlawful entry is a bit more of a gray area; a person who walks into a home through an unlocked door might have made a mistake or be confused in some way, and using deadly force would generally not be justified unless the entrant displays a weapon or makes threats.
Jim
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