Fiendish
Part of speech: adjective
Definition: Sinister; evil; conniving; in the manner of a fiend.
Diabolical
Part of speech: adjective
Definition: Extremely wicked or cruel.Of or concerning the devil; satanic.
Example sentence: The 'Total Information Awareness' project is truly diabolical - mostly because of the legal changes which have made it possible in the first place. As a consequence of the Patriot Act, government now has access to all sorts of private and commercial databases that were previously off limits.