In no specific order, these mildly obscure female serial killers indeed exist. Though females more often poison or kill by sneakier, less bloody means, it's not always the case. Further, female serial killers, sadists, and sociopaths do exist beyond Eileen Wuornos and Elizabeth Bathory.
Vera Renczi - Vera was an wealthy Eastern European serial killer, a woman of means in a higher social class, during the 1920's, who poisoned 35 men, which were lovers, husbands, and even one of her sons. It's assumed that she killed these men to keep them forever. She would place them, once deceased, in zinc coffins to preserve them in her basement. She didn't embalm them, but she did apply a substance to their bodies that solidified their remains and gave them a waxy substance so she could keep them forever!
Dr. Linda Hazzard - Dr. Hazzard was a doctor who performed dietary experiments to cure her patients of whatever ailed them. Her fasting treatments, which often included consuming only broth for months in her clinic, while her patients were unable to leave her clinic and were kept as prisoners. Her patients died invariably, but her "treatments" continued. Further, she would perform her own autopsies and fill out their death certificates, since she was the coroner for her locale. She died during her own self-imposed fast when she became ill. She was finally caught when a patient managed to get help escaping and reported her and uncovered the whole cruel operation. The patients often weighted weights only seen in German concentration camps.
Juana Barraza - Juana was a professional wrestler in Mexico, who was convicted of killing 11 old ladies, but was estimated to actually have killed between 24-49 old women by bludgeoning and strangling them. They were subsequently robbed. Her victims were all over the age of 60, and lived alone. It's thought that Ms. Baraza would pretend to be a social worker and would gain the trust of her victims in this manner. She'd come under the guise of helping them sign up for social programs for the elderly to help them enroll in welfare benefit programs for the elderly.
Darya Nikolayevna Saltykova - Elizabeth Bathory wasn't the only psycho noblewoman who tortured and killed, and she certainly wasn't the only serial killer operating in Eastern Europe who was a woman and a noble. She was notorious in Russia for torturing and killing more than 100 of her serfs, which were most commonly girls and women. Many complaints were initially ignored by the authorities regarding unexplained and strange deaths at her estate. Often those who reported these crimes were punished, as she was a noble who had close ties to royalty.
Finally, though, the families of the murdered serfs brought a petition before Empress Catherine II. Subsequently, the Empress tried Saltykova publicly. She was arrested in 1762, and held for 6 years, while law enforcement investigated her. Officials found evidence of as many as 138 suspicious deaths. She was found guilty of having killed 38 female serfs by means of beating and torture. However, the Empress was unsure about how to punish her. In 1768, Saltykova was chained on a platform in Moscow for one hour, with a sign affixed around her neck, stating that she tortured and murdered. Subsequently, she was imprisonment for life in the basement of the Ivanovsky Convent in Moscow.
Tillie Klimek - Tillie was a Polish immigrant to the United States, who murdered in Chicago, Illinois. She would feign precognitive dreams that predicated the dates when her victims would die. She was accurately predicting their deaths, because she was causing them to die! She would gleefully report to husbands and other that surrounded her of exactly when they would die. She was thought to have killed her first and second husband as well as a boyfriend who followed. She was finally convicted of killing her third husband in 1921.
She even mocked him prior to his death that he would be gone soon. She wore her mourning hat that she knitted herself prior to his death, and subsequently wore at the trial where she was convicted of his murder. She also stored his coffin in her apartment's basement prior to his death. When he met his end, medical professionals suspected arsenic poisoning as the cause of his death, and it was verified, leading to Tillie's eventual arrest.
Her other husbands were then exhumed and found to have been poisoned with arsenic, too, leading to their deaths. Further, she was thought at this time to have killed several family members and
people that resided near her and a dog. It was thought that she killed at least fourteen, and has poisoned even more than that in an attempt to kill them. She died while imprisoned for her crimes on November 20, 1936. That's the day before my birthday!