Mind your own business

Stop policing random people's language. It's not your place to butt into strangers' interactions, no matter how fowl their language and by "storming up to them and demanding things", you certainly won't convince anyone of anything anyway.

While it seems rude for someone to say, who knows what people are like. It’s usually always best to keep your nose out of other peoples business unless you can very very clearly see someone is being bullied or abused. It is nice to stand up for people but try and make sure they need it first.

While you had good intentions, you should stick your nose into others peoples business.
As she was only being called a retard, that something that can be ignored. Also they could be family and just teasing each other.

It's not easy being a social justice warrior.

You have to pick your battles prudently.

Or better yet...f*** off.

At point call them something worse than that one and walk off.

*Triggered* - mind your own business!

You deserved it. Don't meddle in other peoples business.

While I'm all for standing up for someone being abused, bullied etc. this wasn't one of those occasions.

Well, and you are also a moron.

What is a ratard?