Please pay attention to what John said to the Killer in the lounge where they first met.

John said the Killer should have killed his own mother because:
- Since the Killer's mother was a nurse, the police would look at (investigate) him and his family sooner or later and might discovered something against the Killer;
- To avoid suspicion by the police, the Killer killed his own mother, who was a nurse, so that no one would ever relate the serial murders to the Killer.

Correct me if i am wrong.