I dunno if this means he's racist. Jerry Seinfeld is Jewish, after all. I think he lost all good judgement, though. The guy who heckled him was black and he probably had a bad day and wanted to really insult the guy back. Does it make it right? No. But I certainly don't think this makes him 100% racist, especially not since we all are racist to certain degrees. Everyone is, in every race, we all have reservations about other races.
Which is sad, because his career will really be over now. One bad day and a loss of judgement will ruin his life. All people will see is that he's racist. Being racist and saying racist things are two different things. Racist means you act on it constantly, and if he was racist, he wouldn't have worked with Jerry Seinfeld for nine years and be friends with him.
Now, the "This is what happens when you inturrupt the white man" comment does indicate he probably doesn't think much of Blacks, though. Unless he was being sarcastic, but who knows? If your sarcasm isn't obvious, you shouldn't use it.
The news is failing to report that the following night at the Laugh Factory, he apoligized and claims to have made reparations. I hope he did, and at the very least, I hope he learns a lesson from his poor decision.
As for the guy being able to 'take his own medicine,' first off, he didn't make a racist comment at Richards. Second of all, Richards' initial "50 years ago, you'd have been" bit should have been as far as he went if at all. That wasn't racist so much as it was, unfortunately, true. He could have salvaged himself from just that. If you want to heckle back a heckler, you don't resort to race, though. That's contrary to the point and a recipie for disaster.