Any firearm instructor will tell you that a double tap is a time honored method that develops accuracy and ensures your target is down. All that indicates is that she was a trained concealed carry permit holder.
George Zimmerman's instructor didn't tell him that. One shot was all that was needed to save his life.
And the law says force necessary to neutralize the threat -- not to "ensure that the target is down". If that was her intent then that is murder.
A Houston Shell station is not Bagdad.