Even if he felt that he had good reasons, as a Republican, to explore alternative options on the economy and the budget, he should never go out and undercut his Republican colleagues like he consistently does. He should meet with them privately and discuss what he thinks.
That he doesn't do this is very telling. It says that McCain, at best, puts his self-interest above conservatives and his party. Much worse, it suggests that maybe his agenda is contrary to the interests of conservatives and the Republican party. Selfish jerk, or Machiavellian saboteur. It's a scary consideration.
And he takes money from conservatives under the guise of beating Obama, then throws the election and spends their money against a conservative primary opponent.
He acts like the Manchurian candidate that he may well be.