I wish that I didn't think Krugman right about this. I wish we could say that we're experienced an isolated, deranged soul committing a random act of violence. Yes, a deranged soul, but no, it was not a random act of violence. The shooting was aimed at a political figure. Although it came from a deranged individual, the targets were not entirely random--he choose a political target. Someone like this individual, and others before him, act out in a way that they think will resonate in the wider culture.
I also have to say that Krugman is correct when he eschews any pretense that both Left and Right stand equal in invective. The "left", which means, I think, those who adhere to the values of the Enlightenment (and which can certainly include Republicans and conservatives) value reason, democracy, and resist the use of violence. A few--but a very vocal few--on the Right promote fear and violence. Even during the Bush Administration, when from the point of view of many on the Left, our nation's institutions were deeply harmed and our values degraded, we saw no widespread turn to talk of violence and resistance. No, generally in America, violence and fear come from the Right. We see it again. Leaders in the Republican Party need to speak out loudly and clearly against it. Failing to do so may constitute good politics (I hope not), but it clearly constitutes a moral failure.