Disparager

(n.) One who disparages or dishonors; one who vilifies or disgraces.

Thesaurus Entries

Thersites, backbiter, belittler, calumniator, caustic critic, cynic, debunker, decrier, defamer, deflater, depreciator, derogator, detractor, hatchet man, knocker, libeler, muckraker, mudslinger, railer, slanderer, social critic, traducer