> Most people would not say to hide Jews from Nazis was bad faith I think.

That would be perfectly bad faith to the Nazis. There's no such factor as moral good or bad here; bad faith has more to do the intent you have towards each party. If the intent is to explicitly trick someone towards something you want or away from something you don't want, that is usually in bad faith. (There are some exceptions.) If the intent is just to explain something in a way others will understand, even if your explanation turns out to be (knowingly) inaccurate, that can sometimes be in good faith, though I wouldn't call it good practice.