Personally(nothing to do with my Thai nationality), I believe the soul has already left the body and will be subject to Karmic Laws.
Those I aspire to be like, ie religious leaders/saints/Prophets, show compassion and forgiveness. While I can't find compassion for this killer, I try to think he was mentally ill and didn't know what he was doing. Mental illness can happen to absolutely anyone.
Child killers/rapists like Myra Hindley and Ian Brady were cremated after a religious ceremony. I do think they were evil rather than ill. This was in a 'Christian country'. I think it was appropriately done, kept quiet, with few people in attendance.
I think this body should be given a simple ceremony.