One who attacks the reputation of another by slander or libel; SYN. maligner, slanderer, libeler, backbiter, traducer.

ETYM Latin: cf. French détracteur.
One who disparages or belittles the worth of something; SYN. disparager, depreciator, knocker.

Libeler, one who libels, one who slanders, one who maliciously defames

One that uses offensive epithets and invective especially against a political opponent