ROUEN, France — A French prisoner went on trial Monday and acknowledged killing his cellmate and then eating part of the man's lung.Nicolas Cocaign, charged with murder and acts of barbarism, told the court in Rouen, northwestern France, that he is now getting psychiatric treatment and feels "stable." After the charges against Cocaign were read in court, the 39-year-old said he was responsible. Thierry Baudry's mutilated body was found Jan. 3, 2007 by a Rouen prison guard, and an autopsy revealed two chest muscles and part of his left lung were missing. At the time, Rouen prosecutor Joseph Schmit said Cocaign confessed to stabbing his victim with scissors and suffocating him with a plastic bag before removing and eating part of an organ from his chest – though he mistakenly believed it was the victim's heart. Cocaign, whose face is tattooed with images including bloody tears and a skull, told investigators he had wanted to eat the victim's heart to "take his soul," according to the indictment. At the time, Cocaign was serving a sentence for attempted rape while armed, while Baudry was in prison for sexual assault. Defense lawyer Fabien Picchiottino said his client had repeatedly asked to be committed to a psychiatric ward before the killing, but his requests were turned down by authorities. Baudry's mother said she had long dreaded seeing Cocaign in person. "I want to know why he did this to my son," Jacqueline Baudry said. "I want him to pay."Monday's hearing traced Cocaign's life, from his adoption at age three to his run-ins with the law. He risks life in prison. A verdict is expected later this week. A third inmate, David Lagrue, was present in the cell during the attack. Lagrue, who was initially suspected of complicity but was later cleared, committed suicide last year in another prison.