An Indiana teenager who strangled his 10-year-old brother and admired a fictional serial killer should not have been sentenced to life in prison without parole because he was mentally ill, his attorney argued in appealing for a lighter sentence.

Defense lawyer Leanna Weissmann also argued that the judge erred when he let an expert who had never met Andrew Conley testify that the teen might be a psychopath.

"Speculation inferring that a suicidal 17-year-old boy who had never been in trouble in his life is a psychopath from a doctor who never took the time to even talk with the boy has no place in our courtrooms ... and in a decision as to whether that boy will spend the rest of his life in prison," Weissmann wrote in a court brief.

The Indiana Supreme Court will hear arguments on Conley's appeal Monday in South Bend. Conley, now 19, unexpectedly pleaded guilty as his trial was set to begin in September 2010. He was sentenced to life without parole following a five-day hearing before a judge in which his videotaped confession was played.

Conley, then 17, told police he choked his brother, Conner, on Nov. 28, 2009, while they were wrestling at their home in the Ohio River town of Rising Sun. After the boy passed out, Conley dragged him into the kitchen, put on gloves and continued strangling him for at least 20 minutes before wrapping the boy's head in two plastic bags.