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.

She is the police's lead suspect. She couldn't have committed the crime.

She has an unshakeable alibi: 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?