IANAL so i'm not sure where the burden has to fall in this case - but
1) either the defendant has to prove that she was in immediate danger of grave bodily harm and/or death
and she couldn't retreat ... (forensics will say a lot about this)
OR
2) the prosecution has to show beyond a reasonable doubt that what transpired was "merely"
simple assault, for which deadly force was not justified.
without knowing the specifics of the case, it looks like to me #2.
to make it worse: based on the newspaper accounts, she probably violated armed citizen cardinal rule #1 post-shooting: "STFU and lawyer up." What she said to the cops can and will be used against her.