The controversy over his book already feels a little played-out. But I think he’s an individual illustration of one of John O’Sullivan’s laws: that any organization that’s not explicitly conservative becomes liberal over time. McClellan wasn’t an ideological conservative. So I think he was fine with being a kind of conservative as long as he was surrounded (more or less) with conservatives and it suited his career interests. But once you ripped him out of that context and put him among liberal book editors and told him that writing a lefty book would get more attention and sales, he was fine with being a kind of liberal.
My column on the book, fwiw.