Mental illness is Off-putting. Trauma and stress make people hard to be around. There are no perfect victims. If your framework for someone under pressure are people who cry photogenically at home in the bathroom or at night when everyone else is asleep and then wake up and act like they're fine then you will fail to recognize it when your coworker who's normally really nice suddenly blows up on someone for leaving forks in the company sink.
Everyone is going through shit. And it doesn't make it okay for them to treat others badly, but it also sometimes makes it difficult for them to recognize that they're treating you badly, like the person on the phone with 9-11 who no longer realizes that he's telling the operator about his day and not answering questions. When your friend of ten years who has a new boyfriend suddenly starts being a massive bitch to you about your weight maybe she's just being a cunt, or maybe she's internalized some bullshit. You don't have to take that, but you Can go "Hey, what the fuck?" And that is often more helpful than you realize. It is easy to assume that someone who does something cruel is acting with intent, but especially in cases where someone's behavior changed in a short span of time, they aren't, any more than the person who is convinced the beanbag chair is going to fix them.
You don't have to give people endless chances. But you should give them at least one chance. Because on your worst week, it's going to be you crying at your friend's birthday party because she ran out of chili before you got to have some, and you're going to want some grace for yourself.