DI Poole discovers that the "demon" mentioned was not a supernatural entity but a person from the victim's past. The killer had a long-standing motive related to a past family secret and used the distraction of the fête to administer the poison.
Angelique Morel, a local woman who dies during a parish fête. Delitti in Paradiso 1x3
You can find more details and full cast information for this episode on IMDb. DI Poole discovers that the "demon" mentioned was
Initially thought to be a heart attack, the autopsy reveals Angelique was poisoned with cyanide , which was administered via a capsule. which was administered via a capsule.