Maybe she wasn’t. But then again, there is something fundamentally wrong with a grandparent who is prepared to kill their own grandchild, even if said grandchild was a massive scuzzball in his own right. There is something even more wrong when she actually pumps another bullet into him when he is clearly no longer any kind of threat and is weeping into the phone for assistance.
I can’t really feel that there is much room for claiming mitigating circumstances here, unless it can be demonstrated that she was going senile and didn’t really know what she was doing...