The accomplice of "bunny boiler" Sarah Williams reportedly feared the murderer would kill her too by lacing her food with prawns.
Katrina Walsh is said to have believed Williams, who stabbed love rival Sadie Hartley to death after shooting her with a 500,000 volt stun-gun, would frame her as the suspect in a murder-suicide plot.
Walsh's paranoia was revealed as footage emerged of the moment Williams was woken in her bed by police and arrested over the businesswoman's murder.
In letters sent from prison while awaiting trial Walsh, 56, told her ex-husband that she suspected Williams, 35, would trigger her life-threatening allergy with the shellfish and hide her medication before leaving a note admitting killing the mother-of-two.
In one of the notes seen by the Sunday Mirror she wrote: "All she (Williams) needed to do would be to contaminate something I ate with prawns and remove my EpiPens. Then add a 'suicide' note confessing & she'd maybe have got away with this."
Walsh said she believed the speed of Williams' arrest in the wake of the brutal murder prevented her from carrying out the plot.
Then, in a bizarre attempt to bring levity to an apology to Kevin Walsh, 58, she said: "Sorry to mess your life up. Hope you can see the lighter side of it."
The two women were convicted of murdering Ms Hartley, 60, at her home in Helmshore, Lancashire, after scheming for 18 months in an bid to carry out the "perfect murder".
Jailing both for life - Williams with a minimum term of 30 years and Walsh a minimum term of 25 years - at Preston Crown Court, Mr Justice Turner said the "unimaginable ferocity" of the murder of the "loving mother" was like the "slaughtering of an animal".
Williams immobilised Ms Hartley with the stun-gun, then using an eight-inch carving knife to stab the semi-paralysed mother-of-two through the face and neck, inflicting more than 40 stab and slash wounds in the attack on January 14.
The killer's arrest three days later was caught on camera by documentary makers following Lancashire Police at the time.
In the clip Williams is woken by officers storming into her bedroom as she slept at 3am. The killer is seen sitting bolt upright, her face blank.
The footage is due to be broadcast as part of ITV programme The Murder Of Sadie Hartley on September 1.