Why do the Horsemen attack people? So, in Owari no Seraph, we have these weird monsters called "Horsemen of the Apocalypse" or "Horsemen of John", right? 

Why do they attack people? What is their motive? Do they, for example, eat people for nutrition? Are they under the control of the vampires?
No explanation is ever given as to why the Horsemen of the Apocalypse (also know as Horsemen of John) attack humans.

a quote -


  Horsemen only attack humans. Almost as if they were sent to cull the
  world's bloated human population.


Source --Shinoa, Chapter 8, "Mitsuba's Squad

All of them attack humans and only humans, while they ignore every single other entity, they do not attack vampires or animals, with the exception of one Horseman of John, namely - 

Spoiler alert - 


   The special Horseman of John, summoned by Abaddon to destroy the army of vampires and humans