Like a fiend; diabolically wicked or cruel; infernal; malignant; devilish; hellish.
source: Webster 1913
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.