I'm gonna' go with Kenny Kramer on this ("The Real Kramer"), mainly because he offered the most thoughtful explanation about what happened and because he's known Richards for almost two decades.
Basically, Richards is not a standup comic, he's more of a performance comic. So when he started with the hecklers he was already above his head, out of his league. He may have thought he was using "tools" appropriate to a comic versus hecklers in an LA comedy club, buy he crossed the line somewhere.
Given the material and language that's regularly used on late-night Comedy Central and HBO's comedy channel (including use of the "n word" by both black and white comics), I'd have to give Kramer's explanation some credence.
Richard angry rant was racist to the core.
And I don't care how many people stand up and insist Richard's isn't a hateful bigot.