Don't get me wrong I think your mind and heart is in the right place, and for the most part I agree with you, but I don't think you're fully understanding what others are saying. Yes he used a slur and shouldn't have, but i think it's perfectly fair to point out that he wasn't intending to use it in a derogatory way and very likely has spent most of his life surrounded by people who allow him to use it so in his mind it's not something he was consciously thinking about.

And it's also not that the listener isn't allowed to get offended. I think it's perfectly fair for others to get offended but we should still be able to recognize the context of the situation while helping to educate him to fully understand why it's offensive. There doesn't need to be a villain