“I’ve never seen Breaking Bad but I imagined the meth lab would be in this halogen-lit trailer-like setting, but it wasn’t, it was in the middle of this forest,” explains director Matthew Heinemann. “After months and months of trying and getting nothing, finally we got a call: ‘Be in this town square at 6pm and you’re in.’ This group of masked men met us and drove us through smaller and smaller towns, then smaller and smaller fields. They stopped, explained they were just protection and then another car drove us into the lab.”

Through profiling two charismatic but flawed vigilante leaders taking on drug cartels on opposite sides of the US-Mexico border, documentary Cartel Land explores what happens when government institutions fail, when citizens feel forced to take the law into their own hands and the ramifications of what happens when they do so.

In Mexico, José Manuel Mireles lead the Autodefensas or self defence forces in Michoacán state, who from 2012 began beating back the powerful Knights Templars cartel in the region. Across the border, Tim “Nailer” Foley leads the Arizona Border Recon, who attempt to stop the Mexican cartels bringing drugs and violence across the border.

Although cartel violence does spill over into the US, the two movements are operating in very different contexts and the levels of violence between the US and Mexico are incomparable. Seeing their two struggles play out side by side leaves us to question who, if anybody, is really justified in taking up arms.

“It’s really easy to judge in the luxurious confines of a club in London,” Matthew explains. “But I will say, when you’re on the ground, on the border in Arizona, you do feel like you’re in this lawless sort of Wild West world. You don’t really feel the presence, necessarily of law enforcement. You look up on the mountaintops and there are cartel scouts watching you, you can hear them talking about you on the radio.”

Viewers have brought their own prejudices to bear in judgements on the rights and wrongs of each movement. Reactions to the film have differed across borders, with some viewers in Mexico, who previously saw Mireles as one of the few heroes in what is a very bleak situation for the country, disappointed with his warts-and-all portrayal in the film. While in Colombia, some viewers conscious of the damage paramilitary forces had done during Colombia’s own decades-long conflict, criticised the film for glorifying Mireles and vigilantism.

“Some of the rhetorical questions the film asks are, ‘What would I do in that situation? What would you do?’” Matthew explains. “If your sister was raped by the cartel, if your dad was hanging from a bridge, would you take up arms? Is that right? Is that just? Is vigilantism sustainable? These are questions that plagued me throughout the making of the film and that drove me to keep me going down there to understand what was really happening.”

Ultimately, the film presents a highly complex situation and offers no easy answers. At the end of shooting, Matthew was still wrestling with the questions that initially spurred the project. But ironically, the moment he saw it clearest was amid the toxic fumes of the clandestine meth laboratory. “I think the interview with the member of the autodefensas who became a government employee, but was also the meth cook is very emblematic of what is happening down there,” he explains. “The lines between the vigilantes, the government and the cartel became blurred, and in the end they were all one.”

Cartel Land is out now in UK cinemas.