When President John Kennedy visited Dallas in November 1963, he was greeted by a full-page newspaper ad accusing him of being a communist fellow traveler. To his wife he observed, "Oh, you know, we're headed into nut country today." The city, according to historian William Manchester, was a "mecca" for "the Minutemen, the John Birch and Patrick Henry societies."

In the hours following Kennedy's assassination, aides assumed a right-wing radical was responsible. When Robert Kennedy informed Jacqueline about Lee Harvey Oswald's leftist background, she felt sick. "He didn't even have the satisfaction of being killed for civil rights," she said. "It's - it had to be some silly little communist." Eventually, the Warren Commission found no direct connection between Kennedy's assassination and the city's "general atmosphere of hate."

It is a natural human desire to invest tragedy with meaning, to make grief coherent. Manchester, who chronicled JFK's final day, concluded, "If you put the murdered president of the United States on one side of the scale and that wretched waif Oswald on the other side, it doesn't balance. You want to add something weightier to Oswald. It would invest the president's death with meaning."

The killings in Arizona deserve to have a meaning. The first assassination attempt on a female federal officeholder. The shooting of a respected federal judge. The murder of a girl born on a day known for death, Sept. 11, 2001. We want these lives and all the others to be balanced by something weightier than Jared Loughner.

A killer such as John Wilkes Booth represented a conspiracy and a cause. He was hanged to repudiate an idea. A would-be assassin such as John Hinckley symbolizes little more than the sad incapacity of a single mind. Based on current evidence, Loughner more closely resembles Hinckley. Yet he is different in some respects. The alleged Arizona killer shows signs of psychosis. But he also seems to have contributed to his own corruption by dabbling in moral nihilism, conspiracy theories and other drugs. In the absence of organic disease, it is possible for a man or woman to gradually destroy their character and conscience. The voice in Loughner's head may have been his own.

This does not have a crude political application. Some liberal critics of polarization have accused their ideological opponents, on the thinnest of circumstantial evidence, of complicity in murder. This is an extreme and ironic symptom of polarization. But outrage on the right should be tempered by the recognition that many conservatives would be capable of hasty judgments under different circumstances. Suppose an unstable leftist loner, with some peripheral ties to ACORN, had shot a Republican congressman. Americans never use the actions of an individual to judge the guilt of a group - unless it is the Tea Party, or immigrants, or conservatives, or liberals, or Muslims or fundamentalists, or anyone else who is really, really disliked.

Asserting simplistic political blame in the Arizona killings is a destructive enterprise. But a lack of causation does not mean the event is without meaning. There is inspiration in the examples of those who provided aid, including Daniel Hernandez, as self-possessed as a medic on a battlefield. And there is a warning in the example of Loughner. His views are tinged with madness but are not unrecognizable. They are the distorted reflection of any ideology, of right or left, defined by resentment, conspiracy theories, illusions of persecution and hatred for the other. Loughner lives at the center of this blighted ideological landscape; others, from birthers to truthers, visit its outskirts. It is the place where madness and politics merge.

Among the details to emerge from Arizona is Loughner's list of favorite books. "The Communist Manifesto" and "Mein Kampf" do not surprise. But "To Kill a Mockingbird" is harder to explain. It is a moving depiction of empathy - the ability to walk in someone else's shoes. Yet Harper Lee is also brutally realistic about human nature - more realistic than her creation, Atticus Finch.

One hopes that Loughner, with time on his hands, will reread this section: " 'He was out of his mind,' said Atticus. 'Don't like to contradict you, Mr. Finch - wasn't crazy, mean as hell. Low-down skunk with enough liquor in him to make him brave enough to kill children." Crazy, mean as hell, or both, this is the achievement of Jared Loughner.