Don is a complex character which is part of the fascination. He isn't all bad or all good. He has his demons (like many people) and can be reactive like a lot if not most people. I'd argue that a lot of people just react rather than think first before they say or do something.
And I think thats true of all or most of the characters on the show. They're nuanced rather than mostly caricatures.
I, too, agree that not all white people would have been so broken up about the death of MLK. There would have been quite a few like Harry for whom it was just another well known person who died and life moves on. This happens every time someone famous dies---some are really affected and others just shrug and wonder what the fuss is about. We still see this happen---witness more recently when Princess Di and Michael Jackson and Whitney Houston died and they were just celebrities. MLK represented a lot more than the examples I just cited, but the reactions to such a death are still essentially the same.