Use 'devilish' to suggest someone's intentions are hidden and possibly harmful. He whispered a devilish plan into her ear.
Pro Tip 2/3
Morally Dark
'Devilish' can imply that an action or idea is morally wrong and should be avoided. The scheme had a devilish nature, bordering on cruelty.
Pro Tip 3/3
Culturally Sensitive
Be aware that religious individuals might find the term 'devilish' offensive or disturbing. His beliefs made him interpret the movie's theme as devilish.