But what's so bad about being selfish?

It's just another way to act like you don't care. And that you're willing to go along with others being not caring. That's why.

Selfishness is bad for humanity because selfishness is a cruel and unnecessarily dehumanizing way of life that is unsustainable, but it was picked up from others' behavior patterns as a survival mechanism So they could get along better in certain imbalanced systems. 

Some people are taught to be so materialistic and sexist that it feels like in order to be the one who fits in better with them, you're also supposed to go along with routinely devaluing others and making excuses for violent, untrustworthy people.

For example, there was a man who raped a teenager and got so many excuses made for him, and later, some children were killed in a helicopter crash because he was treated like a GOD whose status superceded the weather report the day the helicopter took off.

Maybe if he hadn't been treated like a GOD he wouldn't have felt entitled to rape the teenager or gotten the children killed on the helicopter. But do they act like they care about those human beings? No, they do not.

They acted like this person was their idol, with a higher status than other humans, because they were taught by society to hold up men who act like violent predators, and to be so delusional about the harm this causes, that it makes them seem very untrustworthy.

It's hurtful. They are still choosing to be selfish and uncaring. They still chose being immature and trying to pretend it's not so bad to act that way. It is still perpetuating pain and making the world feel more unsafe. It makes the world more unsafe for other people's children, too.

Like, when that's how they act, how do how do you know if there but for the grace of God go even more agents of harm doing the bidding of a different cult or a lying sexual predator, treating others terribly in exchange for money or social status or a temporary feeling of power?

I just have to figure out how to not let these people upset me so much because it's dysregulating and that dysregulation has physically caused me too much harm and I'm sorry about that.