BIG,
You shoot to stop the threat. That said, ya ain't Annie Oakly, especially in a VERY tense situation. You aim for center mass (biggest target area) and that's it. You do not 'shoot to kill' AND you do NOT 'shoot to wound', you shoot to 'stop'.
Like Haz said, you should not even place any value on the perpetrators life forces, sucks to be him or her. Hesitation kills, don't give ANY credit to the person who is trying to stop your life force, shoot til they can't shoot back, or stab you, or bludgeon you, or grab you or yours. Your mental attitude should be, " f..k these people, who the hell are they to try to take my life, my belongings, or my family " get mad and focused, it will help if you have to be mean. It is also absolutely true. If you or yours are TRULY in danger, you cannot think about the consequences to you, only the consequences if you don't do what needs to be done.
It might mean aggression, and maybe not, but if it does, do it now and sudden and don't hold back. Fighting for you life or someone else's means exactly that, a fight, where 2nd place is a bad deal.