I wouldn?t say sick as such, more that it shows a lack of education, and also shows how society has not evolved in such a way as to make such terms unacceptable to the masses.
As time as gone on it has been considered more and more unacceptable to refer to people of ethnic minorities by the derogatory terms that have been applied to them in the past, but this hasn?t followed in terms of the disabled.
But I do think that a lot of that also has to do with the fact that when you refer to someone as a ?retard? you?re not necessarily thinking of a person with learning difficulties, you?re thinking of the term itself, and how you have applied it to the person you are referring to. Also if you?ve had no experience of someone with a learning difficulty, you might not consider it to be a derogatory term, because you might never have experienced someone being offended by it iyswim. E.g. people have referred to others as ?cretins?, little knowing that cretinism is an actual condition caused by a child being born with a thyroid malfunction which causes stunted growth (form of dwarfism), and severe learning difficulties. But that?s not common knowledge, so the uneducated continue to refer to others as cretins, blissfully unaware of the offence they could be causing.
So I think the answer is education.