We've been having a chat about this in the staff board. The consensus is that arX isn't racist himself (or not that we've noticed) but we feel his use of "Now there's a nigger." was a bit of a faux pas and should preferably not be repeated.
The same would apply even if he had used the alternative spelling of "nigga", or any other variants thereof.
Like Chris Rock says, if you aren't black you shouldn't really be using those terms to describe other people. This may not apply between close friends in real life (sort that out yourselves), but it's a good rule of thumb for online discussion boards and similar situations.
Note that not being white per se is
not the same as being black per se. For the purposes of using the aforementioned terms, anyone who is not actually black becomes an honorary whitefella. Live with it.
If you are black and you want to use those terms, use them at your own discretion.
As an example, the other day Serrated Colon used "niggaz" to refer to some figures in Somalian politics (IIRC). SC is Somalian himself, so I'm not going to take it upon myself to be offended on behalf of all Somalians apart from SC. If any of them want to take it up with him, they can go ahead.
OTOH, if a black person was genuinely offended and SC (purely hypothetically) decided to keep throwing those terms in their face just to troll them, the staff would do something about it. Not because of the word per se, but because he would be being an asshole.
Short version: anyone with half a brain should be capable of being fairly sensible, mature and aware about the appropriate uses or otherwise of certain terms. If you cannot understand this, you may find yourself in a sticky situation.