Heroin addict Garry Kane is accused of battering Kathleen Millward, 87, to death in her home in Stonehouse, Lanarkshire, on January 3.
During the prosecution's closing speech yesterday, the jury was told that Kane carried out a "brutal and senseless" attack on his grandmother, then called his drug dealer instead of calling 999.
Kane claims that he left his gran watching TV on the day she died, and returned to find her lying dead.
But advocate depute Jennifer Bain said that "her beloved grandson" was the only person who might have had a motive to kill her.
She told the jury at the High Court in Dumbarton: "In a pool of her own blood she was left, seriously injured, dying or dead."
Miss Bain said Kane had substantial debts and no income apart from money given to him by his gran, which funded his £40-a-week heroin habit.
She said that Mrs Millward had been locking her purse away.
She said: "Despite the handbag being kept in a locked bedroom, Garry Kane's DNA was on her bag and purse."
Gordon Jackson QC, defending, said the Crown had made the evidence fit, and none pointed towards Kane.
He said that if Kane was guilty, he should get an Oscar.
He said: "The basis of the Crown case is this – he did it because we don't know who else did."
He said police had repeatedly spoken to him, adding: "It is very difficult to keep up that level of deception."
Judge Lord Matthews will now send jurors out to consider the verdict.