Why do people lash out and pass on Pain? I don't understand!

If someone has gone through a tough time and are dealing with some type of trauma, then of course it's understandable that they feel the way they feel. But if they make you feel uncomfortable by always having a go at you or constantly winding you up or acting selfish or just plain difficult, then is it okay that you don't feel okay and that you want to speak up about it? And if someone isn't treating other people in way that you don't think is right, then why don't people say anything? Is it because they know the person is going through a tough time and they know their mental health state is quite fragile? Because I honestly feel like if someone is treating others in a negative way, that's not okay. Because it only spreads the pain and they aren't considering how the other person is feeling.