We see similar dehumanising language in other genocidal actions: it's present in Nazi Germany when they describe Jewish people as subhuman - the same word has been used against people with LD. Not just by the Nazis with their T4 program[1] but also by British eugenics supporters.[2] these eugenic attitudes lost popularity after the war, but they were still easy enough to find.
And apart from these dehumanising effects the word is used to creat fear, alarm, and distress among the people it's used against. People are routinely attacked with that word.
If you think words can't hurt i) you've had a cushy life and ii) you spectacularly lack imagination and empathy.
Really: learn to English and come up with better insults.
[1] http://www.ushmm.org/outreach/en/article.php?ModuleId=100076...
[2] http://www.newstatesman.com/society/2010/12/british-eugenics...
Not to mention that now it seems to be a priviledge of the self-righteous SJWs to proclaim which words should be counted as offensive this week. It never comes from the minorities in question.
You're wrong. Use of the word retard has been strongly rejected by people with a learning disability in the UK. The word disability has also been rejected, which is why I use LD for learning difficulties, or I use specific diagnoses for granularity. They have rejected these terms for many years. The change in language is something they have been driving.