It really is a shame that "retarded" became a word of disparagement and slang for those who are stupid. A word which is widely regarded as insulting more than anything else.

"Moron" was literally something doctors used to describe certain patients once too. Any clinical term used to describe someone of low intelligence (putting aside the difficulties in making that determination in any universal sense) will inevitably be used as a pejorative/insult. It can't be stopped.