First of all, you did not do it by mistake, one word, maybe, but two? Hardly?
Calling your action stupid is not an insult, you can oppose to US Foreign Policy all you want, I have no problem with that, but trying to be a racist and bigot by calling him a "Nigga", it may look cool to you, that does not play well to anyone else. To be honest, I would have called you an idiot for calling him a nigga, you are stupid in oppose to be calling you smart in this sense that you may have an augment before your insult him, but you lost all your point basically by calling him a nigga.
That act is childish and stupid, in oppose to mature and smart post. Hence I don't see any insult for him to call you on reality.