Synonyms for Fiend (same or very similar meaning)
WordNet sense 1 (a cruel wicked and inhuman person):
demon, devil, monster, ogreWordNet sense 2 (a person motivated by irrational enthusiasm (as for a cause)):
fanaticWordNet sense 3 (one of the evil spirits of traditional Jewish and Christian belief):
daemon, daimonFrom the ODE community, based on WordNetadd/edit