Same word as zapper:

aristarch, attacker, backseat driver, belittler, blamer, carper, caviler, censor, censurer, complainant, complainer, defamer, disapprover, disparager, disputer, doubter, fretter, hypercritic, maligner, muckraker, mud-slinger, nagger, nit-picker, panner, quibbler, reviler, scolder, sidewalk superintendent, slanderer, vilifier, worrier, zapper