1) Anti 2) Backbiter 3) Defamer 4) Depreciator 5) Disparager 6) Hatemonger 7) Libeler 8) Maligner 9) Muckraker 10) Mudslinger 11) Pessimist 12) Slanderer 13) Traducer 14) Vilifier 1) Barracker 2) Carper 3) Caviler 4) Censor 5) Censurer 6) Critic 7) Cynic 8) Depreciator 9) Disparager 10) Enemy 11) Faultfinder 12) Frondeur 13) French word used in English 14) Knocker 15) Opponent 16) Reprover 17) Vocal opponent 18) Word-catcher 
De·tract'or noun [ Latin : confer French détracteur .] One who detracts; a derogator; a defamer. « His detractors were noisy and scurrilous.» Macaulay. Syn. -- Slanderer; calumniator; defamer; vilifier.
[n] - one who disparages or belittles the worth of something
disparager noun one who disparages or belittles the worth of something