I'm torn by Kill List. Ben Wheatley's second feature, following the much-lauded Down Terrace, is equal parts impressive and frustrating. With a divisive piece of narrative elusiveness, the third act will either leave you intrigued or irritated depending on how willing you are to embrace the obtuse ambiguity of it.

 Since returning from a disastrous mission in Kiev, ex-soldier Jay (Neil Maskell) has been out of work for 8 months. With his son Sam and wife Shel to provide for, he's lured by his mate Gal (Michael Smiley) to take part in a series of contract hits. Things almost immediately turn strange as their client displays an uncanny knowledge of their past and engages Jay in a bizarre blood pact. As the duo work their way through the kill list, it quickly becomes apparent that Jay has some deep psychological scarring.

Where Kill List excels is as a study of relationship dynamics and psychological trauma. Jay and Shel's relationship is entirely believable, from the natural dialogue to the suburban milieu. Wheatley expertly articulates the strains and stresses of their troubled marriage, where the shift from jovial banter to full blown row happens in the blink of an eye. An early dinner party scene conveys a palpable sense of thinly veiled unrest, before all hell breaks loose. Yet there's no easy blame  - harsh words are exchanged, but they're regretted just as quickly. The other masterful stroke here is the depiction of Jay and Gal's friendship. Again it's rendered with a truthful complexity. As they banter between jobs, you can really believe these guys have been through a lot together, good and bad, not just because they say they have but because you can feel it. The blokey humour is of the best British everyman variety without collapsing into caricature. You laugh with them, not at them.

 As the violence escalates and Jay loses track of his previously professional approach to the hits, we see an altogether more terrifying side to this damaged brain. There are some truly brutal scenes, and more than a whiff of A Serbian Film in their treatment. And when the victims start behaving in a rather unconventional manner in response to the violence, you sense Wheatley's changing gear. He drops plenty of clues throughout as to what might transpire and who may be behind the increasing strangeness - we see unexplained ritualistic symbols drawn in various places, and faith, or lack of, looms large thematically. However, the final act refuses to offer any coherence. Ambiguity can be a great tool for intriguing and some things are certainly better left unsaid, but it's out of place here. The grounding in the real, the quotidian, is what makes the relationships work, it's what makes the hits themselves shocking. The denouement offers a shock which lingers in the context of unexplained teases and, as such, lacks impact. We're no longer sure what's at stake.

No doubt others won't have quite such an issue, and it's certainly an exciting and nuanced work. A doom-laden soundtrack helps, and performances are top notch, but ultimately it frustrates.