Assistant District Attorney Jack O'Neill told the jury that Zysk had committed third-degree murder by repeatedly punching the child so hard that his injuries resembled those caused in an automobile crash.
"What happened is, this man went into that bedroom and took his own frustrations out on a little kid . . . and he caused him to die. That's why he is guilty," O'Neill said.
Third-degree murder, a felony, is punishable by up to 20 to 40 years in state prison.
Defense attorney Evan Hughes told the jury that Zysk was responsible for "an unspeakable tragedy" that amounts to involuntary manslaughter, a misdemeanor.
The night of the slaying, Zysk, his daughter, his former girlfriend, Daniela Gonzalez, 24, and her son Jason were sleeping at Zysk's mother's home in Roxborough.
Zysk and Gonzalez had done heroin and other drugs in front of the children during the day.
"My client was a tired parent who was sleep-deprived, who was abusing illegal drugs" when he struck Jason to stop him from crying, Hughes told the jurors.
He said Zysk hit Jason only a couple of times "just right," but not in a hate-fueled frenzy as the prosecution wants to portray.
"That is a good man who made a horrible, horrible, horrible mistake," said Hughes, who said his client got hooked on drugs in the Navy and met Gonzalez in rehab after being discharged from the service.
"I'm not asking you to let him off the hook. . . . I'm asking you to come back with the proper verdict, which is involuntary manslaughter," Hughes said.
In 2012, Gonzalez pleaded guilty to child endangerment and was sentenced to two years' probation.
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