Bandali Debs gave evidence that Roberts joined him when Sergeant Gary Silk and Senior Constable Rodney Miller were gunned down in Moorabbin on August 16, 1998.
Debs alleged Roberts was the one who fired the fatal shot at Sgt Silk, not him.
Roberts, whose murder conviction was overturned on appeal, denies any involvement in the police killings and has pleaded not guilty in his retrial.
In his closing arguments, Roberts' barrister David Hallowes said it was the prosecution's decision to bring Debs - a four-time convicted murderer - to give evidence.
Debs was not interested in telling the truth under oath, Mr Hallowes said, rather he wanted immunity and to be brought back to Victoria from his NSW prison.
"It gives him a motive to lie, if he even needs any motive," Mr Hallowes told the jury on Tuesday. "He lies for no reason. This is who the prosecution wants to rely on."
Roberts previously told the Supreme Court jury he stayed home the night of the murders, although Debs told him the next day what happened.
In the years that followed, Roberts denied any involvement or knowledge of the police shootings.
In his evidence, the 41-year-old said he lied to police because he didn't want to incriminate Debs or be charged for his role in 10 armed robberies.
Roberts eventually pleaded guilty to those robberies that he committed with Debs.
But when it came to the murders of Sgt Silk and Sen Const Miller, Roberts was merely a person who had been falsely accused, Mr Hallowes said.
"What happened ... was a tragedy," the barrister told the jury in his final address.
"It was a vile and evil act but it was committed by Bandali Debs - a vile and evil person, a psychopath and a liar.
"Jason Roberts was not there. It's time to right the wrong."
Justice Stephen Kaye will start his final address to the Supreme Court jury on Wednesday before it retires to consider its verdict.
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