Hmm i do believe that she was inherently crazy? It all started in the classroom anyway~ Yuki gave her a reason to live and thus she kept her promise to him... in her own way...
But the girl already new, so she wasn't inherently crazy, she was first shaken by whatever affected her before and by the death of Yuki, so she became yandere, I would like to see someone who is actually not brought into being a yandere. I'd also like to see a group of yanderes.
But why couldn't they be born crazy but not crazy enough to be sent to an asylum? Several people can be crazy and still be considered a part of society. Why can't anime character which are born from artist?