First off let me say I hate that word with a passion, it's one of the few words that make me cringe (along with the "N" word) but it seems to be thrown around a lot lately. On social media whenever I engage a conservative and point out how liberal their hero Reagan was and that they would never vote for him today or when I win an argument with a gun nut they often throw out that word or some for of it (like Libtard). Before I mute them so I thank them for it.
While they and most people see "retard" as a negative and while I despise the word I have nothing but love for mentally challenged people. In 2004 after a medical screw up and the several strokes that followed my father lost mental function so it really hits home when someone uses it as an insult.
To be honest most people using this term are full of hatred and self loathing which is 100% opposite of the people they are trying to insult. Most people with mental illness are happy and they don't see their lives as a struggle but enjoy it to the fullest. So go ahead and call me this if you feel the need but realize while I hate the word I take it as a badge of honor and a compliment. I would be lucky to live my life as full of hope and happiness as the people you are trying to insult.