There are plenty of stories from when it happened that pretty clearly suggest that granny had reason to fear. Police had been called to the house before for domestic violence.
Sorry dude, but pumping another round into someone when they have crawled off wounded and are on the phone to 911 isn’t self defence, at that point, it becomes murder.
Sounds to me like she was pi55ed off with him and hadn’t yet finished venting her anger on him for being a drug user and general pain in the butt teenager...