DEVILISH

adj. diabolic(al), satanic, Mephistophelian, fiendish, demonic, cacodemonic, demoniac(al), infernal, hellish, villainous, sinister, wicked, evil, iniquitous, sinful, flagitious, heinous, malign, malevolent, malignant, cruel, maleficent; impish, mischievous, prankish, naughty, crazy, madcap He has come up with a devilish plan for stealing the secret formula