Synonyms for Diabolic (same or very similar meaning)
WordNet sense 1 (showing the cunning or ingenuity or wickedness typical of a devil):
devilish, diabolical, mephistophelean, mephistophelianWordNet sense 2 (extremely evil or cruel; expressive of cruelty or befitting hell):
demonic, fiendish, hellish, infernal, satanic, unholyWordNet sense 3 (morally bad or wrong):
evilFrom the ODE community, based on WordNetadd/edit