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?

PRAISE FOR SOPHIE HANNAH:

'A new novel by Sophie Hannah is a cause for celebration!' ALEX MICHAELIDES, author of THE SILENT PATIENT
'One of the best crime writers currently working' THE SCOTSMAN
'High concept and so satisfying' GILLIAN MCALLISTER, author of WRONG PLACE WRONG TIME
'Brain-bending' GOOD HOUSEKEEPING