Lance Oppenheim's feature debut Primetime, starring Robert Pattinson as a preening television host, premiered at the Venice film festival — and the documentary-maker's first drama-thriller leaves its moral target out of focus.

The film takes as its subject the controversial, top-rated US reality show To Catch a Predator, which ran from 2004 to 2008. Its title evidently borrowed from Hitchcock's To Catch a Thief, itself inspired by the phrase "Set a thief to catch a thief".

The show's method, as the film reconstructs it, was a staged hunt. Researchers baited paedophiles in online chatrooms by posing as children, then lured them to "sting houses", where actors played underage teens on the front porch to draw the abusers inside. There, cameras could film them clearly expressing criminal intentions.

Then came the confrontation. The sonorous host Chris Hansen — the model for Pattinson's character — appeared from nowhere to face the cringing men, and, as the Guardian's review notes, this "made for fantastic television". After each on-camera reckoning, the police made an arrest, and the state prosecutors did the heavy lifting of due process.

That machinery raises the question of entrapment, an obvious issue that, in the reviewer's judgement, the film never raises. It is one of several targets Oppenheim sets up and fails to hit.

The acclaimed documentary-maker has built a reputation in nonfiction, but his move into scripted drama is described as "strangely overwrought and heavy-footed". For all its elaborate neon-lit atmosphere of noir danger, the review finds, the film "isn't at all sure what it wants to say about media or morality, or anything else".

The verdict from Venice is that the real drama — the questions of exploitation, spectacle and justice embedded in the show's format — proves harder to target than the predators themselves. Pattinson's host hunts his quarry on camera; the film around him does not settle on what it wants to say about the media machine that made the hunt prime-time entertainment.

Whether Primetime finds a sharper critical reception elsewhere on the festival circuit will depend on audiences willing to sit with a thriller that withholds a position on the television it recreates.