You know, I'm really not sure how successful seppuku would be in this case. Or why she'd be doing it.
Unless someone around here has a Ghostbusters avatar, and tried to bust Yuyuko. Then there'd be a lot of red haze coming down. And after finding out she couldn't do anything anyway... the only honourable thing left is suicide.
But I'm pretty sure it wouldn't get her very far. She'd just have to kill herself again afterwards. Unless she does that other ghost thing of leaving for the actual afterlife after they've cleared up their business in the mortal world...
And as for what I think of the massacre?
Good riddance to everyone.