The Opposite of Murder

What if the only way you could prevent a murder was by confessing to it?
5.05pm. A terrified Jemma Stelling walks into a police station and says she is about to commit a murder.
She knows that the person she hates most in the world, Marianne Cass, might soon be killed. She knows this because she is the one in danger of killing Marianne, and the urge to do so is growing stronger.
The only way to stop the struggle in her mind is to inform the authorities that Marianne is at risk - from Jemma herself. Desperate to be free of her frightening obsession, she seeks out police officer Simon Waterford and makes what she thinks of as a 'preventative confession'.
5.05pm. Marianne Cass is murdered in her home.
But while Jemma is talking to the police, Marianne is brutally murdered. The time of death window means Jemma is unequivocally eliminated as a suspect.
The problem for Simon is that he's now positive that Jemma Stelling is a ruthless, cold-blooded killer, and possibly the most ingenious one he's ever encountered...