Fiendish, devilish, diabolical, diabolic, demoniac, demonic are comparable when they mean having or manifesting the qualities associated with infernal or hellish beings called devils, demons, and fiends.
Fiendish usually implies excessive cruelty or malignity.
Devilish frequently suggests abnormal wickedness, but it often also suggests superhuman or satanic ingenuity or craft or capacity for destruction. The term is often used as an intensive that substitutes for profanity.
Diabolical often and devilish sometimes connote colder and more calculating malevolence than fiendish.
Diabolic is often used interchangeably with diabolical, but the former term may be preferred when the reference is to devils as individuals of a given character or origin rather than to their malign qualities.
Demoniac and demonic often suggest frenzy or excesses (as of one possessed). More frequently they suggest the inexplicable or superhuman element in life or, especially, in genius.