1
general::
With the intention of deceiving someone or doing harm, as in I'm sure they were acting in bad faith and never planned to pay us. This expression was first recorded in 1631. The antonym, in good faith, meaning "sincerely and honestly," as in I signed that contract in good faith, dates from about 1350.
American Heritage Idioms
2
general::
Phrase(s): in bad faith
Fig. without sincerity; with bad or dishonest intent; with duplicity. • It appears that you acted in bad faith and didn’t live up to the terms of our agreement. • If you do things in bad faith, you’ll get a bad reputation.
McGrawhill's American Idioms And Phrasal Verbs