This applies to fandom but it also applies to other parts of life:
If someone you know has a habit of getting you to dislike or distrust people you had no problem with before
Or if they repeatedly complain about the bad behavior of others but rarely provide receipts (enough objective details about the situation that you can ascertain it for yourself)
Or they want you to always take their side in a conflict, implying or outright stating that if you don't, it's a sign of disloyalty, lack of real friendship, or a moral or ethical failing
Then it's important to consider that this person may not be giving you the whole picture.
Some people thrive so much in the presence of conflict that they purposely pit people against each other in order to get attention and an emotional high.
Before engaging in fandom wank or ostracizing someone in fandom based on secondhand information, consider the source(s) of your information and whether they necessarily have your best interests at heart.