diabolic in WordNet English dictionary

1. extremely evil or cruel; expressive of cruelty or befitting hell
   "something demonic in him--something that could be cruel"; "fires lit up a diabolic scene"; "diabolical sorcerers under the influence of devils"; "a fiendish despot"; "hellish torture"; "infernal instruments of war"; "satanic cruelty"; "unholy grimaces"
   demonic  diabolic  diabolical  fiendish  hellish  infernal  satanic  unholy 
   
2. showing the cunning or ingenuity or wickedness typical of a devil
   "devilish schemes"; "the cold calculation and diabolic art of some statesmen"; "the diabolical expression on his face"; "a mephistophelian glint in his eye"
   devilish  diabolic  diabolical  mephistophelian  mephistophelean