When campaigning for the Philippine presidency last spring, Rodrigo Duterte promised to kill so many criminals that “fish will grow fat” in Manila Bay from feasting on their corpses.

Since taking office on June 30, Mr. Duterte appears to be making every effort to meet that grisly goal. Over 1,800 people have been killed by the police and vigilantes since then, and the wave of killings shows no sign of subsiding.

Many of the victims appear to have been innocent by any definition, and none had been proved guilty in a court of law. But the crackdown has struck a chord with the public, and Mr. Duterte’s popularity has been soaring.

What drives this explosion of extrajudicial violence — which, far from unique, bears striking parallels to previous waves of killings in Colombia, Mexico, Guatemala, Thailand and elsewhere?