Source : Webster's Revised Unabridged Dictionary (1913)

Libeler \Li"bel*er\ (-[~e]r), n.
   One who libels. [Written also {libeller}.] `` Libelers of
   others.'' --Buckminster.

Source : WordNet®

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