A VOLUNTARY worker who sexually abused boys was 'exceptionally dangerous' because he had hidden his depravity behind a facade of helpfulness, a High Court judge said yesterday. Sentencing Ng Kwok-yau, 30, to six years' jail for indecently assaulting four boys aged between seven and 14, Mr Justice Leong said Ng had committed acts of great depravity and used threats to force his victims to succumb to his abuse. The judge did not agree the children had not been psychologically damaged by the attacks, especially in the case of the seven-year-old victim. Ng had threatened to cut off the boy's penis if he did not obey him. It would be impossible to assess the trauma the child would undergo, the judge said. Mr Justice Leong said that while reports showed Ng was not suffering from any mental abnormality, what he had done was exceptionally vile. Young boys should be protected from people like Ng, he said, adding the case's only mitigating factor was Ng's guilty plea, which had saved his victims the ordeal of giving evidence. The maximum term for the offence was 10 years' jail, and the judge said he had used nine years as a starting point and given Ng a one-third discount for his plea. Ng, a former voluntary worker for the Caritas Centre and the Henry G. Leong Yau Ma Tei Community Centre, admitted five counts of indecent assault on four boys under 16. Prosecutor John Mullick told the court the offences had been committed between January 29, 1991 and June 7 this year. Ng had waylaid the children, dragged them to staircases and corridors in Tsuen Wan buildings and abused them. He had forced some of them to perform sex acts and one boy was attacked twice in six months. Clive Grossman, QC, in mitigation, said Ng was not an evil man. He had devoted time to helping people less fortunate with the 'best of motives'. While he was not trying to understate the dreadful nature of the case, Mr Grossman said, there had been no betrayal of trust or abuse of position as there would have been had Ng been a teacher or a scout master charged with looking after young boys. His client had co-operated with the police and was truly remorseful, he said.