Shadow Yuuko isn't the way she is because Yuuko was evil in life, but because she feels hatred for having a wonderful life taken away from her in such a horrific manner.
The Yuuko we know is all sugar and gumdrops because she is as she was without the hatred she feels towards whoever was responsible for her death. Shadow Yuuko, however, is aware of what she lost and is consumed with the desire for revenge. Think about it. Yuuko was trapped and in severe pain, she would never again see the light of day, and there's also a good chance that she may not have agreed with or believed in the practice of sacrificing someone to stop the plague. It would take a saint not to be pissed.- Confirmed, and to make matters worse Yuuko was unwillingly sacrificed by her own sister.
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. Yuuko had the potential to be a Persona User.
A distorted, external version of you that represents the darkness in your heart? Dying in the basement pulled out Yuuko's Shadow, but due to the circumstances (death/ghostly transformation) the Shadow got a little more than it normally would have...