It's hard to know for sure what to make of all of this. One one hand, Occam's razor says a handful of (probably drunk) people who hate McCain did it. On the other hand, the Occam's razor of American electoral politics, if such a thing existed, would suggest that it was intended to look that way and thus is, instead, perhaps the product of shrewd conservatives or Republicans aiming to create sympathy for McCain. In other words: Republican ratf---ing.
If I were betting, I'd put my money on this ultimately being revealed as something similar to the Ashley Todd episode: an isolated act of deranged expression, coordinated neither with the McCain campaign in Virginia nor nationally, but committed by a person or persons who have some screwy ideas about this country.