There is a point to this. While "retarded" in common usage has come to mean "stupid due to mental impairment", that's not really what "retarded" is for. To retard something is to slow it. To retard the spread of AIDS, for example, would be a good thing. We often called mentally impaired individuals "retarded" because they developed at a slower pace- but there was also a cap on their development. Some people will never develop beyond toddler functional levels. That's not retarded, that's stunted.
But there are some people you just need to call "fucking retards", but really, that, to me, isn't insulting enough. They aren't just slow, they're operating in a completely different mental landscape that renders them incapable of navigating the real world like an intelligent human being. They aren't retarded, they're retarted. They're not slow, they are fucking incapable. They are mentally deficient to such a degree that we can't quite understand how they manage to survive this long. They're goddamn retarts.
The etymology here is not, as it seems, the phonetic similarity between "retart" and "retard"- retart, for example, can never be used as a verb. Simply the noun and adjective (retarted) and (arguably) adverb (retartedly) forms. You cannot "retart" something. And that's because of the root word- "tart". This itself implies a double meaning, that of a fruit-filled pastry or a whore. Combining those two images visually is really the intended goal of the use of "tart" as the root. With the "re" before it, as in, to do again, or use again, think instead about an oft-used whore combined with a repeatedly used pastry. This vile image is exactly the estimation one has of a retart's mental capacity.