Insults obviously don't sort issues, but there's a lot of blurring on here between an insult, to vent your feelings, and a challenge of bad behaviour. Taking the PA 'did you mean to be so rude?' or the assertive 'That was rude' as examples, these are not insults, but challenges. Imo doing this in a straightforward and assertive manner is infinitely better than being PA about it. Its clear, its honest and shows courage of conviction. I suppose if your purpose is just to insult someone, it matters less, though to strike at someone who has displeased you through an implication they are of low intelligence, rather than just hit them with the thing you're annoyed by, seems unnecessarily spiteful to me.
I disagree that there's any difference between the 2. Both are explicitly pointing out what was said was rude.
I don't see your version as assertive. Its confrontational. Confrontational is often confused with assertive. It really doesn't open up a dialogue any more than 'did you mean to be rude'. Certainly dont think it shows courage of conviction.
You have attached all these positive associations to confrontation, called it assertive and decided other ways of saying the same thing in a clear manner, is a negative thing.
Both those sentences clearly mean 'I found that to be rude' it's clear you believes it rude.
Again, there's no actual difference in how the 2 statements are being percieved.
So i don't get how one is better than the other.
Also remember that when you say 'that's rude' as a factual statement, you are wrong. Because it's your opinion that something is rude, doesn't make it a fact.