>Perhaps if she would of yelled though the door "Hey I have a gun and know how to use it" when he was ringing the doorbell she wouldn't of had to shoot anyone.

Let's see... "'Don’t answer,' she yelled to her 9-year-old twins playing downstairs."
I'd say he knew someone was inside -- he continued to forcibly enter the house anyway.

Yelling to him that she had a gun (and was ready to use it)
* probably wouldn't have scared him away -- he could take it as an empty threat
* you've just threatened him - which coule likely make him more apt to try to attack ("in self defense")
* to yell through door to give the warning would likely place her closer to the door - a much more vulnerable position for her