I'm not sure self-defense exists in Japan like it does in the US. It's a bit similar to European countries, where it reduces the penalty for the crime, but the crime is still a crime. If she had stopped after hurting one and scaring the others away, she would only have assault charges, which would be entirely negated by self-defense and the fact the people she assaulted were actually assaulting her first. But she did 5 murders, including 4 in cold blood. And this includes the woman who did a lower crime than the others. Justice does not recognize Chinese MC behavior.