I agree with what you say, and I think it is wrong for the parent comment to use such derogatory terms.
That being said, your comment also strikes me the wrong way, and I have received similar comments in real life where I had the same reaction, so I thought a bit about why I really feel that way.
More precisely:
1. the patronizing "you should know", as if your goal was to educate the person, as if not offending others would be a matter to be solved by education, etc.
2. pointing out in a lukewarm and passive-aggressive way that something is offensive. What should the other person do? He clearly knows already, and his goal is to offend. There is no right "not to be offended".
3. that it is not directed at you, but you are offended on other people's behalf.
All in all, while I am on your side, I would find it more heartfelt, if you said "you call me a hick? I'll kick your ass!" then this type of reaction.
I agree that the wording isn't ideal, but I interpreted the commenter you replied to as explaining why the original comment was getting downvoted without real responses. That is a common occurrence on HN when someone steps outside the bounds of civility in non-immediately obvious ways. It serves to give context to both the original commenter, and readers who may wonder why it was downvoted.
Also, why are you so sure that the responder is not offended on his or her own behalf? Hopefully you don't think that just because someone is commenting on HN that they would never be associated with "hicks". I find HN has people of more backgrounds than most folks realize, and as a person with a family who would certainly be categorized that way by many (southern, rural, working-class), I find your insinuation that it would be somehow more "authentic" to threaten violence as far more patronizing and stereotyping than the original comment even.
I was offended on my own behalf. I grew up in a rural, Southern area where I only ever heard 'hick' used by outsiders as a slur against locals. Certainly I have also seen rural people "reclaim" the term, but that doesn't give outsiders the right to use it.
I also find the insinuation that I should have threatened violence patronizing and prejudiced. The parent comment also included an example of how I "should" talk to sound more "authentic". Textbook prejudice.
