When James Wenneker von Brunn murdered Stephen T. Johns at the Holocaust Memorial Museum earlier this month, history was less made than revealed. Officer Johns, a 39-year-old African-American family man, was an easygoing guard, affectionately known to colleagues as "Big John.'' That his last act was to open the door for a member of the public defines his goodness. That von Brunn, an 88-year-old white supremacist and anti-Semite, simply opened fire on the man holding the door defines his malevolence. But more is at work here than an act of lunacy.

Officer Johns was not murdered by coarsened civic discourse, nor by broad currents of racial hatred, nor, for that matter, by inflammatory rhetoric of certain talk-show hosts. Johns was murdered by von Brunn, who should now be swiftly brought to justice. But it is impossible, and would be irresponsible, to ignore the implications of this event: a set of extravagant hatreds combined with a mysticism of the weapon, laying bare multiple connections. A Jew hater, von Brunn could have assaulted a synagogue. A racist, he could have targeted an African-American postal worker. A nihilist, he might have attacked another tourist site in Washington; the guard randomly confronting him might have been white - but what von Brunn in fact did defines its own meaning.

The US Holocaust Memorial Museum affirms that even the darkest human impulses must be reckoned with. The institution itself, including the gravity of architecture that evokes a death factory, epitomizes cultural criticism, imposing on everything around it - from Mall edifices that were partly built by slaves, to the Bureau of Engraving's sanctification of money, to sculpted glorifications of various wars - an unsought verdict of history. Von Brunn attacked such reckoning itself. A denier, he attacked memory.

For all of Hitler's neo-paganism, the Holocaust did not spontaneously spring up out of the Teutonic forest. The District of Columbia may have been carved out of the heart of tidewater slave-holding, but that crime had roots beyond the American South. Indeed, what von Brunn's act dramatizes is that race hatred in Western culture is elliptical, and has two foci: anti-Semitism and white supremacy. In ways that are rarely understood, the former generated the latter, which then curled back as anti-Jewish genocide. Aggression of one group toward others is built into the human condition, but we are speaking of something more deadly than that - an effervescent lethality that is peculiar to the culture that comes from Europe.

What we call "racism'' can be traced to the 15th-century Iberian idea of "blood impurity,'' a biological fault that set Jews apart from Christians. Jewish unworthiness was no longer in their religion, but in their physical makeup - an inherited inferiority. That idea combined at about the same time (1492 a marker) with assumptions of innate European superiority over the "savages'' encountered in first-wave colonialism. The new European imperialism (unlike, say, the imperialism of ancient Rome) depended on the ideology of absolute ranking by race.

The pseudo-scientific idea that "inferior races,'' like inferior species, were properly doomed by laws of nature (survival of the fittest) arrived in time to justify wanton genocide, from Congo to Colorado. Unkilled natives were enslaved. As the scholar Sven Lindquist observed, Hitler's innovation was to apply within Europe, against Jews, the method that conquistadors and colonists had long used against aboriginals on four continents. One thing alone empowered Europeans to wreak such havoc wherever they went, and that was the gun - enabling murder from afar. The gun, in all its forms, was the epochal tool of white male supremacy, which is why it continues to have irrational appeal. As much as the jack-booted hate Jews and blacks, that much they love their guns.

If we humans were condemned to such homicidal impulses by the mere fact of our human condition, then the denial of history would be tolerable, moral amnesia inevitable. But anti-Semitism and racism come from particular times and places, choices and consequences - from culture created by humans. Therefore such culture can be changed by humans - but only if we reckon with its past. It was to history, memory, and the possibility of a better future that Officer Stephen Johns opened the door. May he rest in peace.