In "The Boscombe Valley Mystery," how can the antagonist of the story be the murder victim himself?
Charles McCarthy and John Turner, residents of Boscombe Valley, had lived in Australia at the same time. They met when John Turner and his gang of thieves was robbing a gold convoy that McCarthy was guarding. Turner and his daughter returned to England and built a quiet respectable life for themselves on Turner's share of the stolen gold. Eventually, Charles McCarthy returns to England himself with his son James.
McCarthy finds John Turner and starts blackmailing him saying that if Turner doesn't provide for a comfortable life for McCarthy and his son, he will go straight to the authorities and turn John Turner in; McCarthy could succeed in having Turner imprisoned as a witness to the robbery. In response, John Turner does what McCarthy wants and provides for McCarthy's needs--blackmail demands.
Since McCarthy is a blackmailer, he is the antagonist at this point in the story of John Turner's life because Turner is now leading a quiet retired life doing well and raising his beloved daughter. It happens that McCarthy's latest blackmail demand is that Turner force his daughter to marry McCarthy's son James. James is a good young man and decent but Turner won't have McCarthy blood mixed with his family line. Of course to spare his daughter from an unpardonable forced marriage, he says no to McCarthy's blackmail demand for marriage between the two.
McCarthy goes to Boscombe pond and calls "Cooee," an Australian cry, to summon John Turner, who intends to refuse the blackmail demand again. As Turner approaches, he hears McCarthy and James arguing,with McCarthy demanding James marry Miss Turner. When James leaves, Turner looses his senses and strikes McCarthy with a rock. This is how the antagonist also is the murder victim. The history between them shows the murder victim to be the antagonist and explains the present murder mystery.