Comic violence is difficult to define, let alone do well. Is it a genre in its own right, a sub-set of other categories or simply a style that needs to be anchored, not to say justified, by something worthier of our attention? Ben Wheatley’s Sightseeing, in which Alice Lowe co-starred, got its laughs by associating gore with the caravan, that symbol of mobile domesticity, and by taking to extremes the petty resentments and irritation we all feel but normally keep hidden. Lowe’s debut as director, Prevenge, written, filmed and performed when she was heavily pregnant, treads and sometimes crosses a much finer line, managing to get one emotionally involved then finally losing its nerve.

Lowe’s character, Ruth, murders people gruesomely, the contrast between this and her third-trimester bulge funny but unsettling. Then it becomes clear that she is driven to these acts by the voice of her unborn child. Pregnancy, I am told, involves feeling that one’s body has been taken over, a loss of control exploited by Prevenge for macabre and humorous effect. But even the unborn have motives, and a reason for this prenatal killing spree emerges, retribution as a bloodthirsty form of grief.

Audiences like to be kept off-balance, and Lowe keeps inviting our empathy with her knife-wielding mum-to-be before jolting us out of it with moments of horror and farce. As the film nears its conclusion one begins to wonder which mood will prevail. Might the killing turn out to have been imagined, the tables turned on our laughter and unease? I do not mind being proved wrong, liking a film less for seeming predictable. But the final frames of Prevenge are a missed opportunity, neither thought-provoking nor genuinely shocking, as if Lowe was unsure how to finish what she had so ably started.