I downvote comments that promote hateful ideologies, whether or no they were posted in bad faith. I also downvote posts that derail the conversation, whether or not I think they were posted in bad faith because it is impossible to know if someone is posting in good faith from an individual post. By the time a pattern is clear the thread is derailed.

Context also matters, because the same post about grilled mushrooms as a substitute for grilled steaks will be posted in good faith to different posts and be a net positive or negative depending on the post. A post about grilling in general? Positive, because it adds to the topic! A post about the best cut of beef for grilling? Negative, because it derails the thread to be about not eating beef.

Sure, people should not be downvoting non-important topics or views that they could just block instead. But a lot of people also assume bad faith when someone disagrees with them, so that isn’t good criteria either.

It’s supposed to be for whatever the fuck you want to use it for. There’s no downvote police on lemmy.
Personally, I upvote every reply I get and nothing else.

Ultimately, it’s supposed to be used to make post/comments less visible, for whatever reason.