John McCain is a man of honor. You may not agree with all of his choices. You many not like the guy. But he is a man of honor underneath it all.
He is unwilling to endorse the extreme, and dangerous, rhetoric of the racist right wing of American politics. Given the American history of assassination surrounding the issues of race and integration, he putting a lid on any implied tacit approval of violent measures to stop Barak Obama. His followers needed to hear that.
"Off with his head" is an implied threat of death. We don't usually have that with American politics. I know that people didn't mean "kill Obama" when they said it. But there is an element of violence in that particular expression of opposition.
People have claimed that John McCain has succumbed to the evil methodologies of Rove in an attempt to win this election. And I would have to agree. However, his action in pulling back the rhetoric demonstrates that he still is decent underneath being a candidate.