BOISE, Idaho — 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.

Duncan earlier pleaded guilty in state court to killing McKenzie and Slade and Brenda Groene. Sentencing on those charges is not at issue here.

Prosecutors on Friday also played video clips Duncan made of the children while they camped deep in Montana's Lolo National Forest.

In one, he taunted the children, joking that he would take them home. "Help, they kidnapped me, they won't let me go home or nothing!" Duncan says in the video.

Another clip showed a strange campfire ritual in which he said they were burning their wishes by burning a log covered in writing.

"All of our wishes being burned," Duncan says, off camera. "My wishes for forgiveness, your wishes for..."

"Going home," both children interject.

The video clips did not include segments showing the young boy being sadistically tortured. Federal prosecutors have warned jurors that they will have to watch that disturbing footage.

Duncan, who is representing himself, had little interaction with the court and appeared to nod off at times during the testimony Friday.

Duncan is also suspected in the 1996 slayings of two girls from Seattle who were half-sisters and is charged with the 1997 killing of a boy in Riverside County, Calif.