The lessons of the last 24 hours have nothing to do with whether or not someone ought to be offended about something. If anything, we've learned that there is really no effective way to establish, objectively, what is an isn't offensive.
The lesson we learned was how to handle being offended: to challenge the offender directly, find a resolution, without involving an internet mob.
OP did this the correct way, and you reacted with hostility instead.
"Retarded" is a word that the bullied (and their loved ones) have heard all throughout their lives used in a pejorative manner. It is also rarely used IRL in a non-pejorative context, even though the definition exists. For someone to associate the word with its pejorative and insulting context is not at all unreasonable.
Except that in this case, the "player" is the one who's using certain words in a pejorative context, giving the implication that anybody to whom that word applies is somehow lesser because of it.
This is why we don't use the words "nigger", "kike", and "faggot" as insults anymore, why calling somebody a "pussy" or "woman" for not being aggressive or assertive enough is generally frowned upon, and, yes, why casually dropping the word "retarded" like it ain't no thang will get people for whom the word actually means something politely asking you not to use it anymore.
Communication is a two-way street, and it's up to the listener to determine how they interpret the words coming out of your mouth. I'm amused and irritated by how many people get frustrated when they're told that their choice of language bothers somebody. It's like, hey, here's an opportunity to refine how to interface with the world! And a bunch of people see that opportunity and go FUCK NO, I'M TIRED OF HAVING TO DO THINGS, SOMEBODY ELSE JUST STOP BEING OFFENDED OKAY
This is the internet. We use "nigger", "kike", and "faggot" all the time. Because WORDS.
If you think the correct answer is to try to get everyone everywhere to not use whatever subset of words you subjectively find offensive versus just IGNORING THINGS THAT OFFEND YOU, I'm not really sure what to tell you, other than perhaps:
"We" do not use those words. "We" are not part of a single interconnected culture. You might use those words, since from what little contact I've had with you you come across as a narcissistic asshole with little concern for others. But I am not you, and I thank my lucky stars for that.
The lesson we learned was how to handle being offended: to challenge the offender directly, find a resolution, without involving an internet mob.
OP did this the correct way, and you reacted with hostility instead.
"Retarded" is a word that the bullied (and their loved ones) have heard all throughout their lives used in a pejorative manner. It is also rarely used IRL in a non-pejorative context, even though the definition exists. For someone to associate the word with its pejorative and insulting context is not at all unreasonable.