And then the attack was used to justify an unrelated war...
No, it was not. Bush was very specific that he did NOT suspect Saddam Hussein's involvement in 9/11 on a number of occasions, notably his first speech in the UN on the topic. Moreover, that war - one assumes he is referring to Iraq; not even Krugman would be stupid enough to pretend that Afghanistan was "unrelated" - that war had been going on for a decade before Bush entered office. Whether any of whoever Krugman means by "neocons" did so is impossible to say inasmuch as he declined to identify anyone. That too is a tactic of cowardice.
In fact, the most common complaint against the "neocons" was not that they prosecuted Iraq in the name of 9/11 but that they didn't, preferring instead to couch that activity in terms of advancement of democracy. Krugman is acting as if the silliest accusations on the Left are now established fact merely through repetition. It's not flying, and his refusal to accept comments indicates that he knows it.
Of what, then, is Krugman ashamed? He is ashamed that his country fought back. He is ashamed that it won. He is ashamed that we didn't fall to our knees after 2700 people were murdered and blame ourselves, or at least the Republicans. He is ashamed of his country. He should be ashamed of himself.
Krugman is at the bottom of the barrel with all the other liberals