Ironically, retard was originally a kinder word cooked up to describe people with lower intelligence, along with moron, idiot, cretin, and imbecile. Over time these became pejoratives. Just kind of interesting to see how language develops and terms intended to avoid offense over time turn into ones that offend people.
Yes, when you come up with a word to describe people that others consider less/worse than themselves, they will invariably end up using the word to describe them as an insult to others. This will happen no matter how mundane or non offensive you try to make the word.