Why do people harm others and should we forgive those who have hurt us, directly or indirectly, intentionally or accidentally? As the old theological frameworks have lost their interpretive power, we have increasingly turned to psychology, biology and criminology to best understand the origins and nature of harmful and violent behaviour and how to ameliorate it. Instead of framing harm as an inevitable consequence of humanity's sinful nature, we might instead view it as an aberration of damaged minds, or the product of socio-economic deprivation, or a primeval remnant of our evolutionary past. Nevertheless, the old idea of evil – a malevolent, almost demonic force that drives people to hurt others – retains its interpretive power and there are certain crimes so abhorrent and incomprehensible that we feel only the concept of evil can explain them.

Frozen is a 1998 play by Bryony Lavery that tells the harrowing story of the abduction, sexual assault and murder of a 10-yearold-girl, Rhona, by Ralph, a remorseless serial killer. Through the figures of Rhona's grieving mother, Nancy, and an American psychologist, Agnetha, who is writing a study of serial killers, Lavery asks why people commit violence against others and whether we need to forgive those people in order to grieve effectively and to survive and thrive again after a traumatic event. Agnetha, too, is recovering from the traumatising death of a close friend, killed in an avoidable road traffic accident caused by a stoned lorry driver, and she uses biological psychology to try to understand why Ralph has committed such unforgiveable acts and remains unapologetic. For Agnetha, the origins of murderous acts are to be found in the physical structures of the brain, which can be materially diminished by childhood neglect and abuse to an extent that empathy becomes physically impossible, and she considers Ralph's crimes as symptomatic of illness, rather than evil.

The play opens in 1980, with familiar scenes of ordinary life in the UK, as harried mum Nancy delivers a monologue about her busy family life and the everyday tribulations of childrearing. Nancy's life is upended by the random disappearance of her youngest daughter, Rhona, and we witness the transformation of her life by this one moment, as her marriage falls apart, her relationship with her other daughter is strained, and she is propelled into the spotlight as a grieving mother and a campaigner highlighting the dangers of predatory abusers. In juxtaposition with these scenes, we also encounter Ralph, who never questions the morality of his violent intentions, but instead approaches kidnap, rape and murder with a proud military precision and repulsive sense of entitlement. At first we chuckle at Ralph's angry complaints about the inconveniences of modern life, but the audience soon realises we are in the company of a hateful, violent man who seems incapable of pity or remorse. We are also introduced to Agnetha in the 1990s, as she prepares to leave America and journey to the UK to take up an academic post and meet Ralph as part of her research into the psychology of evil.

Lavery is unafraid to delve into the contradictions and complexities of her characters or to suggest their similarities: she refuses to simply depict Nancy as a saintly victim, or Agnetha as a calm, wise scientist, or Ralph as entirely evil and irredeemable. Nancy, we discover, is something of a Little Englander and we see how her grief transmutes into a harrowing, acidic rage that eats away at her other relationships. Agnetha approaches her subjects and her research with a chilly clinical detachment – and some decidedly iffy scientific ideas that wouldn't have looked amiss in the Victorian period – that is contrasted with her drunk emailing her dead colleague and freaking out on a flight. Ralph, we discover through limited flashbacks of his past, was himself a mistreated child, moulded by parental harshness. All three, we realise, have been emotionally frozen by loss, violence, grief and deprivation. In one of the play's most distressing and moving moments, Nancy visits Ralph in prison and gently but powerfully forces him to use empathy, possibly for the first time, to appreciate that he inflicted great suffering on her daughter. Forced to confront his guilt, Ralph commits suicide, while Nancy, we sense, can begin to emerge from the fugue of bafflement and hatred that have scarred her life for over twenty years. With its focus on relationships between and across generations, Frozen explores how the hurts that are inflicted upon us can emotionally freeze us and engender repetitive compulsions, in which we unconsciously re-enact and relive the traumatic moment.

Such a difficult piece depends on strong, convincing characterisation and this production delivers memorable, striking performances from the three leads. Ruth Sullivan, as Nancy, delivers a stunning, utterly convincing performance as an ordinary woman forced to confront the unimaginable. Sullivan is impressively adept at portraying the shifts in Nancy's character, as she struggles with the terror of her daughter's disappearance, the horror of the discovery of her brutal murder, and the long years of grief, rage and, ultimately, an acceptance that approaches forgiveness. In some of the play's most distressing and tender moments, as when Nancy recalls cradling the "beautiful" skull of her disinterred daughter, Sullivan powerfully commands the attention and sympathy of the audience. As Ralph, Michael Bettell gives a fearless performance, never flinching from depicting the horrifying, violent aspects of Ralph's character with utmost fidelity. At first, affable and ordinary, Bettell delivers Ralph's series of unsettling, almost emotionless monologues with great commitment and chilling believability. Making her Tower debut, Lisa Rost-Welling is dexterous at depicting the conflicting sides of Agnetha's personality, as she meets Ralph, and delivers her findings to a lecture hall, with bland professionalism, while also struggling with her grief and loneliness. The characters shift between monologues and dialogue, which, in the final scenes feel almost intrusive in their truthfulness and poignancy. The three leads deliver what is often very difficult material with great skill.

Director Jacqui Marchant-Adams must be commended for having evinced such powerful performances from her leads and the production overall is adroitly directed. Some of the transitions between the scenes rely a little too heavily on cross-fading, which slowed the pace, and the rigorous physical separation of the characters, except during scenes of dialogue, could have been softened. The impressionistic design sits a little at odds with the play's stark realism, but it's an interesting and bold choice. On the night I saw the production, several audience members were visibly moved and there was a deserved standing ovation from some of the audience for this bleak but insightful and moving tale of loss and redemption.