What if the only way you could prevent a murder was by confessing to it?

Jemma Stelling has confessed to a murder.
She is the police's lead suspect.
She couldn't have committed the crime.
She has an unshakeable at the time of Marianne Cass's brutal murder, Jemma was at 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?