The issue is, you’re using a disability as a weapon. You’re saying “it’s bad to be retarded. You’re retarded. Therefore you’re bad.” The very act of using that term as an insult necessarily insults all people who are themselves medically retarded. In short, if being retarded wasn’t a bad thing, why would you use it as an insult?
I can see that as a legitimate intent. Unfortunately, intent is often hard to convey, without like, what, 4 posts worth of comments on a social media thread? I don’t really care about the word myself. I just wanted to highlight how, even without malicious intent, you can end up hurting someone you don’t want to.