1) Befitting Beelzebub 2) Devilish 3) Diabolical 4) Evil 5) Extremely evil or cruel 6) Fiendish 7) French word used in English 8) Mephistophelean 9) Mephistophelian 10) Outrageously wicked 11) Rather wicked 12) Wicked Found on https://www.crosswordclues.com/clue/diabolic
diabolic 1. Very wicked or evil; fiendish. 2. Showing the cunning or ingenuity or wickedness typical of a devil, or Satan. 3. Having the qualities of a devil; devilish; fiendish; outrageously wicked; such as, a diabolic plot. Found on http://www.wordinfo.info/words/index/info/view_unit/277/
having the qualities of a devil; devilish; fiendish; outrageously wicked: a diabolic plot. · pertaining to or actuated by a devil. Found on https://www.infoplease.com/dictionary/diabolic