In Shaul Schwarz’s “Narco Cultura,” Buknas de Culiacan celebrate drug violence from its base in California, while in Juarez, Mexico, the body count skyrockets. (Shaul Schwarz)

The documentary “Narco Cultura” tells two stories. Or, more accurately, it plays two contrapuntal melodies.

One tune is set to the polkalike beat of the narcocorrido — a jaunty, accordion-based Mexican folk song that has been updated for a contemporary audience with lyrics drawn from the thug lifestyle of Mexican drug cartels. Director Shaul Schwarz’s film focuses on one of the narcocorrido genre’s young, rising stars: Los Angeles-based singer-songwriter Edgar Quintero of the popular band Buknas de Culiacan. A characteristic snippet of the group’s Spanish lyrics translates as, “We’re bloodthirsty, crazy and we like to kill.”

One particularly disturbing shot shows Quintero singing these words, almost like a lullaby, to his young child. Both seem oblivious to their meaning, let alone to the deeper implications they hold about society.

The second melody spun by the film is more funereal.

Schwarz’s film cuts back and forth between Quintero — who seems to have a pretty nice life in California — and the depressing world of Richi Soto, a busy crime-scene investigator in the Mexican border town of Juarez. As Soto tells it, in a somber, almost dirgelike recitation of statistics, the number of drug-related killings in Juarez keeps climbing, now reaching well over 10,000.

By the end of this troubling film, the cognitive dissonance that it highlights — between the theoretical glorification of the illegal Mexican drug industry and its actual cost in blood — is jarring. It’s an important film, but “Narco Cultura” is also maddeningly hard to watch.

Quintero compares narcocorrido music, aptly, to gangster rap. The fact that some places in Mexico have tried to ban the genre only seems to have made it more popular. One delicious irony is the fact that American Wal-Marts carry narcocorrido CDs, despite the chain’s occasional practice of banning music by acts it deems offensive, such as Marilyn Manson.

It’s easy, the film argues, to imagine that a Mexican drug lord is like Robin Hood, when the only evidence you have is his musical legend. Quintero is shown composing a song, on commission, to honor a criminal.

It becomes not so easy to see things this way when you spend a little time with Soto, who’s kept so busy — and who solves so few cases — that he and his colleagues are referred to, derisively, as “bullet collectors,” for their habit of amassing forensic evidence that almost never leads to a prosecution, let alone a conviction.

The Mexican and Mexican American consumers of narcocorrido music are seen as silly enablers of a sort, insulated from the horrors of the cartel violence by a curtain of denial. Although Schwarz doesn’t shy from pulling back that curtain, there’s another consumer whose appetites even more directly enable violence, and about whom “Narco Cultura” makes nary a peep.

That’s the end user of such illegal Mexican narcotics as marijuana, cocaine and crystal meth, most of which are flowing into the hands of buyers in the United States. After watching “Narco Cultura,” it’s hard to imagine any “recreational” user of pot wondering whether the joint he or she just smoked isn’t soaked in human blood.

★ ★ ★

R. At the West End Cinema. Contains grisly images, obscenity, drug content and nudity. In English and Spanish with subtitles. 102 minutes.