Weird news - Hong Kong - Man kills his parents and then cooks them

Sometimes you read about real weird things and wonder as to how they can really happen. This is one such case. Killing other humans happens on a regular basis, with people doing murders for a number of reasons - these could be gang related, these could be because of strong emotions such as anger, hate, frustration, jealousy, etc. However, it gets more rare when a person kills his / her family members, especially parents; although even that is not unknown. However, things get really weird when somebody kills their parents, has no clear reason and then gets rid of their bodies in such a crazy manner. Imagine killing parents, then removing their heads and cooking their bodies and placing the cooked parts along with rice in lunchboxes. For somebody who is investigating such a crime, it would be difficult to comprehend whether they were indeed going on the right track, with such an unusual and barbaric crime (link to article):

An elderly Hong Kong couple were allegedly dismembered by their son and his friend before being salted, cooked and packed into lunchboxes "like barbecued pork", a court has heard.

The severed heads of 65-year-old Chau Wing-ki and his wife Siu Yuet-yee, 62, were found in March last year, stuffed into two refrigerators in a bloodstained apartment, days after they were reported missing.

The other remains were found in a rubbish bin, cooked and packed into lunchboxes with rice, in an attempt by the duo to hide the crime after they ran out of space in the refrigerators.

How does one even sentence such a crime ?