But the forces that animated him, as evidenced by his inflamed manifesto — ignorance, intolerance, bloodthirsty tribalism — are ancient. The Internet and social media did not invent or refine evil; they just made it accessible on demand, in all its banal and lurid manifestations. As for the suspect’s evident wish to instigate discord and sow divisions — he wrote that he wanted to “incite violence, retaliation and further divide” and hoped that by carrying out his massacre with a firearm he would add fuel to the United States’ gun debate — he’s a little late.

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Still, it’s critical that world leaders clearly and precisely denounce this ghoulish act. An attack on mosques, as on any place of worship, is especially sinister and dangerous. Online racists lionized the murderer as a hero and cheered his killing spree as he streamed it live. In fact, he is a monster who slaughtered innocent people — parents and children, the old and the young.

President Trump is not to blame for the tragedy, despite his own history of Islamophobic statements and a travel ban that targets predominantly Muslim nations. Still, he should go further than he has; for starters, by condemning the alleged killer, whose nativist rhetoric — he called immigrants “invaders,” attacked “mass immigration” and wrote that he hoped to “directly reduce immigration rates” — overlaps with the president’s own. On Friday, Mr. Trump cited an “invasion” of immigrants to justify his national emergency declaration to build a wall along the U.S.-Mexico border.

And Mr. Trump, who could not bring himself to criticize the white nationalists in Charlottesville who chanted that minorities (Jews, in that case) would “not replace us,” on Friday said he doesn’t regard white nationalism as a problem. That’s the wrong message. Instead, he ought to state unambiguously that the New Zealand suspect’s “replacement” ideology is an unacceptable trope in civilized discourse.

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It remains to be learned how the suspected killer got his hands on the semiautomatic weapon he used to mow down victims. New Zealand’s gun laws are more lax than those in his native country, Australia, and the massacre at the mosques in Christchurch has already inspired calls for tighter regulation. That debate should be paid heed by Americans, many of whom are by now, sadly, inured to mass killings.

Our hearts ache for the victims and their families; for a community of Muslim refugees and migrants who were welcomed with open arms in a new home that may now look threatening; and for New Zealand, a U.S. ally where the hideous tableau of firearms violence has not been prevalent before now.