While assuming good faith is a fundamental principle on Wikipedia, it generally isn't helpful when you get angry at someone who doesn't know how Wikipedia really works. Therefore, when an editor insists that what they're doing is an improvement when it isn't, assume no clue before assuming bad faith.
Instead of assuming that fellow editors are out to harm the project and its ideals, assume that they don't know how they're contributing in a non-constructive way. Assuming no clue preserves sanity and helps communication: instead of accusing someone of harming the project, you can help them contribute constructively.
If an editor just added material into a Featured article on a film that is sourced to a fan blog site, don't assume bad faith. Assume that she or he does not know about Wikipedia's policy on Reliable Sources.
If another editor just reverted a direct quote you made from the Oxford English Dictionary regarding the etymology of the article name with the edit summary "dubious-never heard that origin of the word before", do not assume bad faith. The editor may genuinely believe that folk etymology is the best source for word origin, rather than Wikipedia's approach, which is to prefer etymologies that are found in sources published by reputable publishing houses.