Is it revealed what kind of race our victim was, or where he was from? He sold stuff from Klatch, but that's supposed to be a parody of Arabian countries, so his name (within Pratchett world) would be (vaguely) Arabic as well. I would rule out he's Klatchian, on that (thin) basis.
Also, was it mentioned what was stated on the pamphlet? Was it a membership drive? Or a "get rid of foreigners" thing?
So, we have a man who can be either the victim of the Hippos, in which case he should have been either a foreigner, or somebody who opposes how foreigners are treated by the Hippos.
Or, he was a member of the Hippos, and was hit in the head by somebody of the opposing side. The marbles thing doesn't suggest anything within the Discworld Canon, but anything with stones would suggest either Dwarves or Trolls to me.
My other options (cults, SK) do not seem to fit this one case.