BOISE (AP) — The man who admits kidnapping two northern Idaho children and killing one of them wrote a letter to his mother blaming society for feeding the evil in his heart, a prosecution witness testified Friday.

"I have once again become a medium of violence in the world," Joseph Edward Duncan III wrote. He said he had been "inflicted by an evil 'demon' that is nurtured by our so-called Criminal Justice System."

Duncan, a convicted pedophile from Tacoma, Wash., pleaded guilty in December to federal charges related to the kidnapping of young Shasta Groene and her brother Dylan in 2005. A federal jury is hearing testimony on whether he should be sentenced to death or life in prison without parole.

The letter to Duncan's mother was read to jurors Friday by Idaho State Police Detective Fred Swanson. Apparently never sent, it was found folded in Duncan's coat pocket after his arrest.

"I don't know what God wants for me, I just don't know," Duncan wrote.

The children were taken from their Coeur d'Alene home in May 2005 after Duncan fatally bludgeoned their mother, Brenda Groene, their 13-year-old brother, Slade, and the mother's fiance, Mark McKenzie.

Both children were sexually abused before Duncan shot and killed 9-year-old Dylan at a Montana campsite. Shasta, then 8, was rescued July 2, 2005, when a waitress spotted her and Duncan in a Coeur d'Alene restaurant.