[DontSueMeBro]

I'm not the person you asked, but I agree with their post.

It's often hard to have this discussion without it devolving into an argument over "political correctness" [0] so hopefully that can be avoided.

Easiest answer is that the term retarded originally was an acceptable term for people with disabilities, but now has a negative connotation, and when used pejoratively as OP did carries the (possibly unintended) implication that people with disabilities are of less value.

I think Wikipedia entry on Retard (pejorative) [1] is a good summary.

It's also worth mentioning "ableist language" and "ableism" here. [2]

[0] - https://en.wikipedia.org/wiki/Political_correctness

[1] - https://en.wikipedia.org/wiki/Retard_%28pejorative%29

[2] - https://en.wikipedia.org/wiki/Ableism