Zysk, 30, broke down and sobbed once during testimony in which he insisted he did not know that the toddler, Jason Larkin Jr., was seriously injured after Zysk punched him two or three times in the side.
"If I thought he was seriously hurt, I would have immediately called 911," Zysk testified.
Despite admitting to the punches that killed Jason, Zysk accused homicide detectives of "foul play" in the interview in which he confessed.
Zysk told the jury that detectives misled him about his need for a lawyer and persuaded him to confess to punching the 40-pound toddler up to eight times to match autopsy findings.
Assistant District Attorney John O'Neill questioned why Zysk waited to raise allegations of coercion, and noted that Zysk initialed and signed every page of his statement verifying it was true.
"I didn't want to rock the boat with this detective," Zysk replied. "I was scared."
Under questioning by O'Neill, Zysk also acknowledged that he lied in his first interview with detectives in the Special Victims Unit, telling them that he never touched Jason and that he thought the boy was moaning because of a stomach ache.
The U.S. Supreme Court has ruled that police detectives can mislead suspects while questioning without violating their constitutional rights.
Still, defense attorney Evan T.L. Hughes argued that homicide detectives "did not play by the rules," and urged the jury to find Zysk guilty of the lesser felony count of involuntary manslaughter.
The Jan. 15, 2011, incident occurred in the Roxborough home of Zysk's mother, where Zysk was then living. That night, Zysk's girlfriend, Danieala Gonzalez, 24, and Jason were staying over.
In his statement, Zysk said he and Gonzalez used heroin earlier that day. He said he was tired and could not get to sleep, and grew increasingly frustrated because Jason would not stop crying. Then, about 2 a.m., Zysk said, he "lost control" and hit Jason because the boy continued to cry.
According to the autopsy, the punches to the side pushed the boy's liver against his spine, cutting the organ and causing him to bleed to death.
O'Neill argued in his closing that the fatal injuries, with other bruises on Jason's body, proved malice and warranted a verdict of third-degree murder.
"For all the rest of life that made him feel like a loser, he could still beat on that little kid," O'Neill said of Zysk. "No one was there to stick up for that kid that day, but you're here to stick up for him today."