I suspect Krugman’s incivility is a deliberate strategy. And this passage from an old Krugman post is why I think so [emphasis Krugman’s]:

“I’m not saying to turn the other cheek and always say something polite as a general principle; by all means lash out at your critics, if you have something to gain by doing so. Rudeness at the proper moment can serve a purpose — as I hope I’ve demonstrated over the years. But if you vent for the sake of venting; if you alienate people you’re going to need; then you’re just being stupid.”

So, when you see Krugman pinning murder on Tea Partiers, or getting nasty, don’t ask yourself, “Why can’t he argue like an adult?” Ask yourself, “Why is he choosing not to argue like an adult?”