Warning: The following contains SPOILERS for Batwoman, season 1, episode 15, "Off With Her Head."
Kate Kane broke the biggest rule Batman has for vigilantes in the latest episode of Batwoman, strangling the mad Dr. August Cartwright to death. This game-changing moment was the capstone of an intense interrogation, during which Cartwright attempted suicide.
An original character unique to the Arrowverse, Dr. August Cartwright was the architect of Beth Kane's misfortune and largely responsible for the trauma that transformed her into Alice. Dr. Cartwright found Beth after she was washed downstream following the death of her mother and held her captive, forcing her to become a companion to his son, Johnny. He also forced Beth to aid him with his experiments in crafting realistic flesh masks so that Johnny, whose face had suffered severe scarring, might take on a normal appearance.
"Off With Her Head" revealed that Dr. Cartwright was not alone in tormenting a young Beth Kane, introducing the character of Mable Cartwright; Dr. August Cartwright's mother and the Queen of Hearts in Beth's Alice In Wonderland inspired fantasies. An overbearing and vain woman who was dependent on an oxygen tank to survive, Mable Cartwright moved in with her son shortly after Beth began answering to the name Alice. Mable treated the teenage Alice like a slave, burning her hands when her tea was too hot and beating her savagely whenever anything was not to her liking.
Kate Kane learned all of this after Alice all left Dr. Cartwright tied up for her next to the Bat-Signal, along with a note suggesting she "Ask about Mommy Dearest." It was revealed that Alice hadn't been referring to Mable Cartwright and that Dr. Cartwright's atrocious behavior went far beyond his abducting Beth Kane and allowing his mother to enslave her. Dr. Cartwright confessed that he had also recovered the body of Kate and Beth's mother, Gabi Kane, and severed her head, intending to give her face to his vain mother before he realized she was a prominent figure in Gotham City who would be immediately recognized. He pacified Mable, however, by giving her Gabi's garnet earrings, whose gems matched the necklaces Kate and Beth had been given at their Bat mitzvah.
It was the discovery of her mother's head in a refrigerator in Dr. Cartwright's lab and the realization of why Mable's earrings looked familiar that pushed Alice over the edge and led to her first kill, roasting Mable Cartwright alive with an improvised flamethrower made from her own oxygen tank. The revelation had a similar effect on Kate, who had just learned that Dr. Cartwright was also responsible for killing the innocent and untraumatized Beth Kane of another world in the mistaken belief that she was Alice. Kate began strangling Dr. Cartwright, declaring that he "stole everything" from her. By the time Kate realized what she was doing it was too late and Dr. Cartwright, already weak from blood loss after attempting to slit his own throat, was gone.
It will be interesting to see how this changes Kate's image of herself in the coming episodes of Batwoman. It certainly seems to have changed the dynamic between herself and Alice, with whom she began plotting how to hide Dr. Cartwright's body at the end of the episode. While Kate is unlikely to hang up her cape and cowl, she's bound to wonder if there's really that much difference between her and her sinister sister now.