The video, captured by a security camera in Charlotte, N.C., shows a 23-year-old woman named Iryna Zarutska sitting on a light-rail train one night in late August, dressed in the uniform of the pizza parlor where she worked.

She is looking at her phone when suddenly, a man sitting behind her stands up, gripping a knife in his raised right hand. Moments later, the police say, he stabbed and killed Ms. Zarutska, a Ukrainian refugee, in what appeared to be a random and unprovoked attack.

The police arrested Decarlos Brown Jr. soon after and charged him with first-degree murder. But the brutal killing did not capture widespread attention until the security footage was released on Friday, at which point it became an accelerant for conservative arguments about crime, race and the perceived failings of big-city justice systems and mainstream news outlets in the Trump era.

The outrage over the Charlotte killing is a part of a pattern in which President Trump and his allies highlight horrific crimes to bolster their case that the country is plagued by “American carnage,” as Mr. Trump put it in his first inaugural address, despite statistics that show crime is dropping. In Charlotte, overall crime was down by 8 percent in the first half of this year compared with the same period last year, according to the police, while violent crime was down by 25 percent.

On Saturday, Representative Mark Harris, a Republican who represents parts of Charlotte, called the attack “a microcosm of a national epidemic.” On Monday, the White House called Mr. Brown, who the authorities say is homeless and mentally ill, a “deranged monster” with a “lengthy rap sheet,” blaming the murder on local Democrats and accusing them of being soft on crime.

“It’s the culmination of North Carolina’s Democrat politicians, prosecutors and judges prioritizing woke agendas that fail to protect their citizens when they need them the most,” the White House statement said.