A 15-year-old boy who killed his mother and sister because he believed there were too many people in the world was ordered to be detained indefinitely in a psychiatric hospital, a Hong Kong news report said on Thursday.

Kan Ka-leung used a cleaver to hack to death his mother and younger sister, later telling an ambulance officer that fewer people in the world would be more environmentally-friendly, the South China Morning Post reported.

Kan, now 16, pleaded not guilty to two charges of murder, but guilty to manslaughter claiming diminished responsibility, which the prosecution accepted.

Reports presented in court agreed Kan was schizophrenic and he was ordered to be detained indefinitely in a psychiatric hospital at a hearing on Wednesday.