"I was getting frustrated. It was a long day, I was tired. That's no excuse," he said while being questioned by defense lawyer Evan Hughes. "I lost control for a split second and I hit him a few times. I felt so bad."
Zysk said he then picked up Jason and took him to the bedroom where he and Gonzalez had been sleeping, and where they had been doing drugs.
When Jason continued to fuss, Zysk said, he took the child back to the room where he slept. He said he later felt remorseful for having hit the child and went to check on him.
"I felt so terrible about what happened. I was trying to ask him if he was OK, and he was unresponsive," Zysk said.
The blows that Zysk landed on the 40-pound child lacerated his liver and led to his death by blunt-force trauma and internal bleeding, according to the medical examiner.
Although trauma injuries were found on the child's nose, lip, head and hand, and bruises were found on his hip and chest, Zysk denied causing those injuries.
"I hit him a couple of times on the side and the stomach area," testified Zysk, who alleged that homicide detectives coaxed him into giving a statement in which he said he struck Jason more times than he actually did to account for the multiple injuries.
Defense lawyer Hughes, in his closing argument, told the jury Zysk made a mistake, is remorseful and should be convicted of manslaughter, a felony that carries a maximum sentence of five to 10 years in state prison.
"This is a momentary loss of control. A horrible, horrible mistake," Hughes told the jury.
Assistant District Attorney Jack O'Neill argued that Zysk lied about not causing all the injuries and that he brutally battered and murdered the boy.
The motive, O'Neill said, was Zysk's frustration with the state of his life: having to live with his mother, frequent fights with Gonzalez, his inability to get an erection and being addicted to heroin - which he ran out of that night.
"He beat that kid to death, now he wants a break from you," an emotional O'Neill told the jury.
He asked them to "stick up for" Jason and convict Zysk of third-degree murder. The more serious felony carries a maximum sentence of 20 to 40 years in prison.
The jury of seven women and five men is set to begin its first full day of deliberations this morning.
On Twitter: @MensahDean