Wellll… it’s not just the root word itself, but how it’s used. Note that the clinical term “mentally retarded” really means “slowed down in progress”, a somewhat neutral way to describe what’s happened to someone, and it’s not a noun, not necessarily an encapsulation of the entire person (unless so used in a sentence). Make it a noun, call someone a “retard” or “-tard”, and suddenly you’re summing up their entire being; that’s what really makes it insulting.