MANILA—Jimmy Vosotros was found dead along a highway on the Philippine island of Cebu last month, his hogtied body wrapped like a mummy in garbage bags and packing tape and a rope around his neck.

A message left at the scene invoked this country’s man of the moment: President Rodrigo Duterte. “I’m a bandit, Du30,” it said, using common shorthand for the man driving a law-and-order push.

The...