(n.) One who backbites; a secret calumniator or detractor.

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