A Mystery?

 Sherlock Holmes explains why he believes Professor Moriarty murdered his victim:  "Elementary, my dear Watson. Professor Moriarty wanted to inherit his grandfather's estate and the millions of pounds that went with it, but he believed his grandfather's impending marriage would mean the new wife would inherit everything.  So the only way to ensure the inheritance was to kill him before the marriage with a poison that no one could detect -- well, almost no one."

Dr. Watson, who is usually accepts his friends reasoning, but who is a medical doctor, objected this time:  "How naive, my dear Holmes.  Several neurons in  neural cortex  of the brain of Moriarty fired which cause an electrochemical impulse to be propagated in the nervous system of the murderer which contracted muscles in the arms and hand that opened the poison and poured it into the tea cup."

Which one of these detectives (and their explanation) is correct?  Could they both be correct -- or maybe neither?  What is the best way to explain human behavior?