She said Roberts was that person.
Gordon said it was clear Karlos died as a result of trauma that caused bleeding on the surface of his brain and behind his eyes.
Medical experts had explored shaking, an impact to his head or some kind of abuse including slamming and punching, she said.
The force required to hurt Karlos was similar to falling from a height or being in a car crash.
There was no evidence Karlos had suffered an "accidental" injury and it was "clear" he had been a victim of abusive trauma, she said.
Gordon argued that on November 29, baby Karlos was suffering flu-like symptoms and Stephens was under the impression Roberts would take Karlos to the doctor. However, he did not return the baby until the next morning, telling her "Karlos is not breathing".
Gordon said Roberts had wanted to bring the baby to his Alison St home and call an ambulance because he wanted emergency services to believe the baby became ill in that home.
This was because of the "mess" and "trails of vomit" left at the Homedale St home he took the boys too, Gordon argued.
Gordon said Roberts had created a "trail of lies" to cover up what had happened, including an alibi of being out drinking until 3am the day Karlos died.
She said the defendant may have hoped Karlos would wake up but he was "guilty of death and guilty of murder".
Roberts' lawyer Simon Lance warned the jury to be cautious when relying on Stephens' evidence.
He spoke about her methamphetamine use, her mental health issues and how she was "prepared to give up her twin boys to a man she barely knew", after meeting him only three weeks before.
She was suffering "significant mental illness and stress" at the time, had fallen out with her mother as a result of her behaviour and had trouble "bonding" with her twin boys, he said.
It all painted a "disturbing" background picture leading up to the weekend Karlos died.
Lance said the narrative became "hazy" after 5pm on November 29. Stephens suggested Roberts took the boys about 9pm but a defence witness suggested Roberts had been with her until 3am without the boys.
He said Roberts had come back early in the morning to Alison St to find Karlos very unwell in the care of Stephens.
He asked the jury to remember previous defence evidence of Stephens barricading the door and "strangling" another of her sons, he said.
An Oranga Tamariki worker had told the court the twins had been "beautiful" and "healthy" in the care of Roberts and they had a special "bond", Lance said.
No scene photographs or forensic evidence linked Roberts to Karlos' death and all allegations of lying could have been Roberts trying to protect Stephens, he suggested.
He said the Crown's idea of shaking the baby did not line up with "reckless" murder and the idea of slamming and punching was highly unlikely.
He said the jurors needed to put their emotions to one side and "focus only on the evidence".
Justice Sarah Katz will sum up the case tomorrow morning before the jury is sent to deliberate.
