While the film presents a complex web of secrets and violence, ultimately the evidence points to Madame Blanc as the mastermind behind the murders. Here's why:
* Her Motive: She is obsessed with a dark, ancient force known as "The Three Mothers," and she believes the academy is the conduit for their power. She kills to maintain the academy's sinister status and her own power within it.
* Her Powers: She is shown to possess supernatural abilities, including manipulating the environment and influencing people's minds.
* Her Connections: She is directly connected to the other witches who are involved in the murders, and she uses her authority to control them.
* Her Actions: She is seen performing rituals and using dark magic, suggesting her involvement in the murders.
It's important to note: The film intentionally leaves some ambiguity, and some might interpret other characters as also playing a role in the murders. However, the most concrete evidence points to Madame Blanc as the orchestrator and main perpetrator.
A final thought: The film's ending reinforces this. When Susie Bannion finally confronts and defeats Madame Blanc, the academy collapses, signifying the end of her reign of terror.