[Maédhros]

I think you'd have a hard time finding an "evil to the core" anything in the books. All of the examples of evil we see in LoTR have been taken and twisted by some greater power. Gollum - twisted by the ring; the Nazgul - twisted by the nine rings; orcs - twisted by Sauron/Morgoth; Wormtongue - twisted by Saruman; Saruman - twisted by Sauron. The only "pure" evil in Tolkien's work is the original evil of Melkor/Morgoth, and perhaps a few other creatures of the Silmarillion and their offspring (Ungoliant & Shelob, for example). All creatures seem to be equally capable of evil, although not all creatures tend to have equal tendencies towards evil.