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McCain didn't hold back from "hammering" Obama with Wright out of a sense of honor or fairness. His campaign realized that it had worked for a moment but wasn't going to work anymore, wasn't going to work with the voters they needed (it had already worked with some, but not enough). It was a purely political decision.
And yes, there was violence in Vietnam that was not of our creation. Does that mean the incredible violence that was of our creation was somehow a good or justifiable thing? You really have to twist yourself in knots to believe that, IMO.