Frink just explained it to you. The fact that retard is interchangeable as an insult with stupid and idiot is what makes it offensive. You’re basically calling mentally retarded people stupid idiots.
The problem being that nobody I know calls them mentally retarded. It’s a dated term. Perhaps older generations, or legal documents, or people in certain regions still use it, but it is fast falling into complete disuse.
facepalm
Why are you guys bending over backwards trying to defend the use of retarded as an insult? If it’s a dated term, there’s even less reason to use it as an insult. And calling yourself “retarded” is still an insult, you’re just insulting yourself.