JOHANNESBURG—On the morning of May 25, a volley of gunshots rang out in Constantia, Cape Town. The suburb is home to some of South Africa’s wealthiest people, including business magnates and politicians. 
It was also home to a married couple who a neighbor later described as “very quiet; from somewhere in Europe.” 
No one knew much about them; they kept to themselves, was the common description of the man and woman whose bullet-riddled bodies—along with those of a man later described as their bodyguard and a woman who worked as their housekeeper—were found in a black luxury SUV in the driveway of their mansion. …