She was neither.
What the legend says: It is claimed that one day she slapped one of her maids so hard that she got blood on her hands. Noticing that the blood made her skin smooth and soft, she arrived to the idea of bathing in virgin blood to keep herself young forever. She started kidnapping girls, torturing them, and bleeding them out for regular baths. Eventually someone reported her to the authorities and she was captured and brought to trial. She died walled into her own home.
What really happened: It is biologically impossible to bathe in blood. It was a political trial. She was a widow with a large estate, strong political connections, and the king himself owed her family tremendous amounts of debt. She was brought to trial, but never actually convicted; all the witnesses confessed under torture, and she herself was never questioned. Her alleged accomplices were beheaded. They sent Erzsébet into house arrest, walled her in, and she died five years later in miserable conditions.
History is more cruel than legend sometimes.
But why medicine?
There is a theory supported by a re-examination of the trials that suggests that there were indeed bloody things going on in Erzsébet's basement - she was trying to heal people. If you are familiar with medicine in the 16th century, you know that never looked pretty. Many things mentioned as her "sadistic torture" methods actually line up with the medicine of the time: Ice-cold baths, bloodletting, etc. As the lady of the land she felt responsibility to care for her subjects. The bodies allegedly buried in the castle garden were probably the victims of illness, but the marks of attempted cures made people think they had been tortured.
Still not feeling bad enough for her?
Báthory was known for taking in girls, regardless of their ethnicity or social standing. She housed and sheltered girls that had been disowned, raped, or widowed, and made efforts to help and educate them. She essentially ran a women's shelter in her own home. This probably gave fertile ground to the allegations that she was gathering virgins to kill, and made the work of her accusers a lot easier...
(I have a full-hour storytelling show based on her legend and her life. I feel like the myth of the "Blood Countess" has been told enough)
Aslo, if you watch one movie about her, make it this one (warning: blood and boobs, obviously)