maligner

from WordNet (r) 3.0 (2006)

maligner
    n 1: one who attacks the reputation of another by slander or
         libel [syn: {defamer}, {maligner}, {slanderer}, {vilifier},
         {libeler}, {backbiter}, {traducer}]

from The Collaborative International Dictionary of English v.0.48

maligner \ma*lign"er\, n.
   One who maligns.
   [1913 Webster]