Madam Justice Beeson added that Tang's life was marked by a lack of responsibility, refusal to support himself by getting a job and tinged with the belief that the 'world owed him a living'.

The court heard that on the morning of July 5 last year in the Tsuen Wan flat they shared, Tang was confronted by his grandmother, who asked if he had been paid.

But Tang had not gone to work for several days and had spent all his time at a games arcade.

As she sat on the edge of her bed folding joss papers, Tang smashed a porcelain vase across her head, causing the woman to collapse to the floor.

After clearing away the fragments of the vase and mopping up the blood which gushed from his grandmother's scalp, Tang wrapped her head in a towel to stop the bleeding.

He only returned to bind the woman's hands in case she awoke.

A pathologist found the most likely scenario behind the cause of death was that the head injury had caused a concussion.

'Because of a lack of proper care, her condition deteriorated due to [profuse bleeding] and breathing difficulties from an awkward posture and she was not able to recover consciousness and eventually died from complications of the head injury.'

Madam Justice Beeson said she 'found it difficult to believe that a street-wise 17-year-old would not have understood the difficulty his grandmother was in'.

'An anonymous 999 call would have been enough for his grandmother's safety.'

Defence counsel Kevin Egan told the court the boy deeply regretted the incident and described the case as 'tragic'.

The court heard the boy was abandoned by his mother when only a few months old and his father remarried and left him in his grandmother's care when he was seven.

Mr Egan argued for the crime to be taken at the 'lower end' of the manslaughter scale. But Madam Justice Beeson rejected this.

She said she arrived at this conclusion after finding the attack was over a trivial matter and that there was gross negligence on his behalf in not calling out for help.