The Opposite of Murder

What if the only way you could prevent a murder was by confessing to it? Jemma Stelling has confessed to a murder. Jemma Stelling is the police's lead suspect. Jemma Stelling couldn't have committed the crime. Jemma had an alibi. At the time of Marianne Cass's brutal murder she was in the police station confessing to a murderous obsession with Marianne. Is Jemma Stelling innocent? Or is she an ingenious cold-blooded killer? Can you be guilty of the opposite of murder?