Agnes Magnúsdóttir was born on 27 October 1795 in Iceland, during a time of rural isolation and social hardship. Not much is documented about her early childhood, but by her 30s, she had built a reputation as a bright, imaginative, and capable woman. Though descriptions of her appearance vary, several sources agree she had a strong and distinct personality.

Agnes worked as a maidservant across various farms in the Húnavatnssýsla region. Around 1828, she was employed at Geitaskarð, where she met Natan Ketilsson, a charismatic farmer and herbalist known for his affairs with local women. Agnes was reportedly drawn to Natan and later began working at his farm, Illugastaðir, hoping the relationship would become something more serious.

However, those hopes were dashed when Natan chose a 16-year-old maid named Sigríður Guðmundsdóttir as his housekeeper instead. This decision created jealousy, rivalry, and emotional tension between the women and between Agnes and Natan. Adding to the complications, Friðrik Sigurðsson, a young man interested in Sigríður, also became involved. Agnes allegedly hoped that if Sigríður chose Friðrik, Natan would return to her. But nothing worked in her favor.

On the evening of 13 March 1828, a tragic and violent crime unfolded at Illugastaðir, a remote farm on the Vatnsnes Peninsula in northern Iceland.

According to historical accounts, Friðrik Sigurðsson arrived at the farm and had both Agnes and Sigríður hide in the barn while the two male occupants, Natan Ketilsson and his guest Pétur Jónsson, slept inside the baðstofa, the main room of the house. Once the men were asleep, Agnes and Friðrik entered the room, and Friðrik stabbed both men to death with a knife. It's unclear whether Agnes took part in the physical act of killing, but she was complicit and present throughout the event.

Before fleeing, the baðstofa was set on fire, likely in an attempt to hide the evidence. Sigríður reportedly stole valuable items before the fire was lit, though her direct involvement in the killings remains uncertain.

In the early hours of 14 March, Agnes appeared in the nearby town of Stapakoti, claiming that Illugastaðir had caught fire accidentally and that both men had died in the blaze. Locals quickly traveled to the scene, but when the flames were extinguished and the bodies were recovered, it became clear that the fire had been a cover-up for murder. Both victims had been stabbed, not burned alive.

Agnes, Friðrik, and Sigríður were arrested shortly after. A full investigation followed, and Agnes and Friðrik were both found guilty of the double murder. Sigríður, though involved, was not sentenced to death.

Agnes was sentenced to death by beheading, in accordance with Danish law, which governed Iceland at the time. On 12 January 1830, she and Friðrik were executed together at Vatnsdalshólar, in Austur-Húnavatnssýsla, before a gathered crowd. She was 34 years old.