Hartley was killed at her home in Helmshore, Lacashire in January this year after she opened the door to find Williams there with a 500,000-vault stun gun, reports The Independent.
She was paralysed in the hallway of her home as Williams preceded to stabbed her with "demonic savagery" before being left in a pool of blood.
"Sarah Williams, over a period of about eighteen months you plotted the murder of a woman whose only crime was to love the man you wanted for yourself, " said Justice Turner, according to The Telegraph.
"But let no one make the mistake of thinking that this was a crime of passion. It was a crime of obsession, of arrogance, of barbarity but, above all, it was a crime of pure evil. You Walsh, are every bit as morally degenerate as Williams. No wonder you have been the best of friends over so many years."
Williams and Walsh have both been sentenced to life in prison and will serve a minimum of 25 and 30 years respectively.