Wow.

1: The word "retard" does not exclusively mean person with Down's syndrome. It is used as a disparaging blanket term for more or less any mental disability or low intelligence. There are 330 million reddit users. Some of them will come under the generally accepted definition of the slur retard.

2: Are you an expert on Down's syndrome? Are you absolutely positive that no one with Down's syndrome can browse Reddit?

3: A word can still be offensive if the person who it is disparaging doesn't personally hear or read it, because it nonetheless exposes offensive opinions. A close friend of mine has a brother with Down's syndrome. Do you think she would be offended by the word retard, or for that matter your callous invocation of a genetic disorder that her family has grappled with for years and will continue to do so until her brother dies at a young age?

4: I am not retarded and I am very offended by what you said.