Bellatrix LeStrange is more than mean, she would be considered as almost as evil as the dark lord himself. She is evil due to the fact she belives what she is doing is right. Between murder and torture she is indeed very very very mean but also a very well written character. She not only murders with out care but according to Dumbledore "she plays with her food before she eats it." She is supposed to be borderline psychotic (especially in the films) because Azkaban destroyed her.
She was sent to Azkaban for torturing the Longbottoms, Neville's parents, into madness using the Cruciatus Curse. Using it is very difficult unless you enjoy inflicting extreme pain. And she did this long enough to destroy their minds. So she was already psychotic before she got to Azkaban.