People who use words do so for a particular purpose. That’s what I mean by design. The n-word had one and only one purpose: a humiliating slur against a group of people.
Since this is obviously not the case with the word “retard” or “moron,” etc., I find the comparison obtuse at best and bad faith at worst.
Ultimately, people will use terms to call each other stupid. This is inevitable since people are, in fact, stupid.