(n.) One who rails; one who scoffs, insults, censures, or reproaches with opprobrious language.

Cassandra Eeyore Thersites backbiter belittler calamity howler calumniator caustic critic debunker decrier defamer defeatist deflater depreciator derogator detractor disparager hatchet man killjoy knocker libeler malist man-hater misanthrope misanthropist misogynist muckraker mudslinger negativist nihilist pessimist prophet of doom railer retreatist seek-sorrow sexist slanderer social critic traducer woman-hater worrywart