I think this is the answer...
The evil Donna Killick
Donna Killick (Fiona Bell) is the hidden villainess of the sixth season of BBC's shetland
Donna was the head of an animal sanctuary and the mother of Alan Killick, who worked with her at the sanctuary. Her backstory revealed that her husband Kevin (who died prior to the episode's events) was abusive to her, which led her to have an affair with Duncan Hunter; the father of DI Jimmy Perez's stepdaughter Cassie.
The series began with Thomas Malone being released from prison for the murder of Lizzie Kilmuir, having served a twenty-three year prison sentence after proclaiming his innocence. Soon afterwards, though, journalist Sally McColl (and daughter of Donna's lover, former police officer Drew McColl) was found murdered in a similar fashion to Lizzie: strangled to death and found in a kiln. Sally was Alan's girlfriend, and while Donna initially appeared upset by Sally's death, she was later revealed to have been disapproving of Alan's relationship with her (much to Alan's frustration).
Over the course of Jimmy's investigation, Kevin was believed to be Lizzie's killer after familial DNA of Alan's was found on the scarf used to strangle her. The speculation left Donna irate, especially when Jimmy had her husband's body exhumed to determine if he was the killer. Donna also grew disapproving of Alan spending time with and offering sanctuary work to Cassie, putting her at odds with her son. After uncovering old footage of a beach party Lizzie attended, Jimmy learned about Donna's affair with Duncan and made a shocking realization: Alan was a product of the illicit relationship, thus making him Cassie's half-brother.