A Roxborough man accused of punching and killing his girlfriend's son because he wouldn't stop crying went on trial for murder Tuesday, with the prosecutor saying Joseph Zysk took out his personal frustrations on the body of a 3-year-old.

"This man here murdered that little boy. That's what this case is about," Assistant District Attorney John O'Neill said in his opening statement to a Common Pleas Court jury.

O'Neill said Zysk, 30, admitted to detectives that he punched Jason Larkin three or four times. The punches pushed the 3-foot, 40-pound boy's liver against his spine, cutting the organ and causing him to bleed to death, O'Neill said.

Defense attorney Evan T.L. Hughes said the case did not involve murder, but rather "an awful, horrific, unspeakable tragedy."

Hughes said the evidence would show that Zysk was guilty of nothing more serious than misdemeanor involuntary manslaughter.

"That's no monster over there," Hughes said, pointing to Zysk. "That's a good man who unfortunately made a horrible, horrible mistake."

Hughes said Jason's death Jan. 15, 2011, was the culmination of years of personal problems for Zysk, including a decade of heroin addiction that began while he served with the Navy in the Iraq war, a failed relationship with the mother of his 3-year-old daughter, and his relationship with Jason's mother, Danieala Gonzalez, 24, a heroin addict he met in rehab.

The night of the incident, Zysk, a railroad lineman living in his mother's Roxborough house, was hosting Gonzalez, her son, and his daughter, who was visiting him that night.

O'Neill said Zysk and Gonzalez had been using heroin earlier that day in a car with the two toddlers present. That night, Zysk became angry when the boy - who lived with his father five days a week - kept crying and went into the child's room, and he hit him several times, O'Neill said.

Homicide Detective John Harkins testified about the statement Zysk allegedly gave him the day after Jason's death.

In the statement, Harkins said, Zysk told him he grew frustrated because Jason wouldn't stop crying. He said he "pushed down real hard" three or four times on Jason's chest. When Jason turned on his side and kept whining, Zysk said, "I kind of thumped him down on his side with my fist" three or four times.

"After I hit him, he stopped crying," Zysk's statement continued. "He was injured. I didn't mean to hurt him, and I feel bad. I hugged him, and I told him I was sorry I hit him."