Morally speaking, I've worked with violent felons. In this situation, as described, an escalation to potentially lethal use of force is excessive. The woman had a range of possible responses. In her shoes, I'd have gone indoors, closed my door, and called the police. I have zero problems with appropriate use of force. In this case, however, I feel her actions were excessive. Furthermore, it was premeditated shooting. She PLANNED to shoot this kid after longterm harassment. She could similarly have phoned the parents, called the police, requested a restraining order, or asked for mediation or victim's assistance. Or all of the above. She was 68, which is not 80, nor is it elderly.
Yes, the boy was a bully and probably a delinquent little thug. However, out of a wide range of options, this woman chose to escalate this longstanding feud to a potentially lethal level.
I'm not a fan of using a hammer when what's needed is a screwdriver.