It doesn't get more comprehensive than this:
Your suspicion of Charles Krauthammer's pre-war arguments is not justified. Some reasons:
1) Mr. Krauthammer has discussed the dangers of WMD and terrorism for a very long time and has done so when many pundits and foreign policy experts focused on other issues (the economy, China etc.) In his famous essay on America's "Unipolar Moment", he foresaw the importance of these two issues at a time when the Cold War had only just ended. He has brought up the WMD issue again and again, and Iraq was - naturally - one of the countries he was always most concerned about. Why, after being honestly concerned for years, should he suddenly have stopped to believe in the danger - while still proposing a war that he no longer deemed necessary for security reasons? That would make no sense whatsoever.
2) Mr. Krauthammer is an outspoken man, and he is a convinced unilateralist. Should he consider war against a certain country as necessary in view of important American interests, I am sure he will say so, and without much ado. I really don't have the impression that he is a man who needs false justifications to argue for what he believes to be in the best interest of America.
3) Mr. Krauthammer is a proponent of what he calls "democratic realism". While that technically makes him a neoconservative, the "realist" element has dominated most of his positions for the most time. He has never been an extreme "idealist" or "moral crusader". The war in Iraq made sense for a "democratic realist" only if there was a strategic reason for it. Iraq's importance for the regional balance of power may have been part of that logic, but I don't believe for a second that it would have been enough to make Mr. Krauthammer a proponent of this war, if not for the danger of WMDs. As for ousting Saddam and democratizing Iraq - Krauthammer is way too much of a "realist" to support a war of such importance for "idealistic" reasons alone.
4) Why (and how) should Mr. Krauthammer of all people have known what countless foreign governments, international agencies, former members of the Clinton administration, independent analysts and even Saddam himself did not know or even deem possible at the time? And what good reason is there to believe, keeping in mind this administration's problems with security leaks, that Mr. Krauthammer would be perhaps the only person to know about the government's deceptions to this very day?
5) There also remains the question of why the administration should have used the WMD argument if they knew it to be a lie. There was no way this would not have been found out afterwards, and it should have been clear that this could only hurt American credibility. The only way to prevent this would have been to carefully plant some WMDs in Iraq, which obviously didn't happen either. And IF the administration had decided to use a pretense for war, it would have been a much smarter idea to trumpet some more alleged evidence of Saddam's links to the attacks of 9/11. They would have been easier to fake and much harder to disprove than the claims of Iraqi WMDs, they would have created an even greater sense of urgency and they would not have allowed for a negotiated solution or inspections of any kind, making war much more probable.
6) Simple common sense: Regarding the overall accomplishments of the Bush administration's foreign policy: Is it more plausible that the WMD rationale was a careful, politically smart and successful attempt at deceiving the whole world - or that is was one more example of sheer incompetence?
Those are just some reasons that come to my mind why I don't believe that the Bush administration, much less Charles Krauthammer, intentionally lied about the existence of WMD. Is it impossible that they did? No. But I'd much rather go with Occam's razor.
Incompetence is indeed perhaps the most plausible answer. I'm going to re-read parts of "Fiasco" and "State of Denial" to sum up my worries with more detail. The Krauthammer omission (which could have been an oversight) is trivial compared with the underlying issue. An invasion plan without a serious contingency plan for finding and securing WMD sites cannot be described as an "oversight." Given the risk posed to U.S. troops, it's not just incompetence either. It's criminal incompetence or outright deception. Stay tuned ...