Seen from behind the glass partition of a Texas prison, Michael Perry, 28, flashes a chipmunk smile and wears an incongruously relaxed manner. "I'm a Christian," he says, "I believe that paradise awaits."

A good thing too, because, in a mere eight days, Perry will be strapped onto a gurney, injected with lethal serum, and thus executed by the state. Into the Abyss, the latest Werner Herzog documentary to venture down the dark recesses, tells his story – and others too, in an effort to reignite the capital punishment debate.

By Herzog's lofty standards, the result is mildly disappointing. The film lacks the sociological depth of The Executioner's Song or the emotional wallop of In Cold Blood. But it sure is a surpassingly, and compellingly, strange tale.

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Of Perry's guilt, there is no doubt. Using police footage, the doc revisits the bloody crime scene and recounts the details. Ten years previously, the teenage Perry and his accomplice, Jason Burkett, set out to steal a car and, in the process, shot to death a woman, her son, and another boy. Openly boasting of their deeds, they were soon cornered and, after a shootout, arrested, whereupon Perry immediately confessed.

That much is clear. So is Herzog's stance on the issue at hand. He uses no narration, but his voice is heard conducting interviews, at one point unequivocally stating: "I believe that human beings should not be executed."

Nevertheless, ample time is given to the victims' family members, who hold up photos of the loved ones while displaying their own deep psychological wounds: "My world was ripped out from beneath. Our lives are empty." While all this is undeniably affecting – the killer facing his imminent death, his extended victims facing their life-in-death – none of it does much to inform or advance a familiar debate.

Matters grow more edifying when Herzog shifts the focus to Burkett, who was equally culpable but got off with a life sentence. Why? His father, a career crook serving a 40-year sentence of his own, explains that he testified at his son's trial, telling the court of his parental neglect: "I was as much at fault as he is."

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Reduced to tears by this admission, two female jurors defied the majority and voted to spare Burkett – proof that the hard law of capital punishment can be randomly soft in its application.

Unfortunately, Herzog offers no interview with Perry's remaining parent, his middle-class mother, and her absence leaves a gaping hole in our understanding of this condemned man.

However, the director does delve into the trailer-trash habits that seem to predominate in Curnoe, the small town where the principals resided; where shootings and stabbings and drugs and a professed love of Jesus are as common as Jack Daniels with a Bud chaser; and where even homeless teenagers own a gun or three.

In such a place, the bizarre is commonplace, which, of course, is largely why Herzog is there – bizarre has been the fodder for his entire canon. But, here, the very strangeness of the characters sometimes deflects our attention from the serious debate – characters like the prison chaplain who waxes lyrical about the God-given beauty of his favourite golf course; or like the weepy former executioner who, after "over 125" completed jobs, has changed his song to "No one has the right to take another life."

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Still, none is stranger than Melyssa Thompson-Burkett – yes, Burkett's current wife, who met and married him in prison, who has known him only as a convicted murderer, and who, after hinting at some outward-bound smuggling plus artificial insemination, points to her belly swollen with the fruits of their love. Then she too holds up a photo, not of the dead, but of the unborn: It's an ultrasound image, a fetus wiggling on a cellphone.

Into the Abyss