Dusk Maiden of Amnesia/WMG

=== isn't the way she is because   but because she feels hatred for having a wonderful life taken away from her in   === The Yuuko we know is all sugar and gumdrops because she is as she was without the  And think about it. It would take a saint not to be pissed.

Shadow Yuuko is, in part, the deity of the shrine over which the school was built
Yuuko was sacrificed to bar the deity. Said deity was already enraged at the treatment it had received, having it's shrine obscured and obliterated. When it saw Yuuko being senselessly sacrificed, it agreed to take on all of the emotions and memories that tormented her upon her death, but they did nothing to improve the already outraged deity's disposition.