I think there's a difference between people who complain- as in- 'I'm unhappy about this'. And those who are angry: 'I'm unhappy and it's your or someone else's fault. Why aren't you doing more to help me? Why isn't anyone? This has upset me but it's unjust and I'm angry (with just about everyone) for it.'
Truthfully, the first I tend to have more sympathy for. Even if they are angry about their overall circumstances, they aren't so aggressive towards others.
The latter- 'I'm owed' attitude can come across like a personal attack on others. I tend to have a bias though, because I grew up with a (suspected) narcissist who was continually playing the victim and accusing/ blaming me for things I hadn't even done. So, it tends to be a trigger for me when people start blaming or expecting things of others. Or, when they get very defensive or offensive over things.
Obviously though, we do have a right to get angry over unjust or offensive things. Plus, we should be pointing it out to one another if we'd made an offensive or questionable remark. I think it's how we do it though. Some people seem to be deliberately argumentative, confrontational, provocative- to try and initiate an argument. I really don't have time for all that. I can feel myself getting worked up by it. Even if it's about someone else. There's that fear they'll turn that aggression on you. So, I just try to avoid people like that.
It's not to say they can necessarily help it. Maybe shit's happened to make them so aggressive. Still, I just feel like shit's happened to me to make me feel like I don't cope well being around them.
It can't be nice to feel that angry really. They must genuinely feel persecuted to lash out at people. So, I sort of feel bad for them too. Just, not enough to want to be around them.