In adapting Pumla Gobodo-Madikizela's book, based on her interviews with one of apartheid's chief killers, Nicholas Wright has chosen to dramatise a subject hard to stomach for many suffering the atrocities which shape modern history.
In her attempts to understand the nature of evil and its relationship to vengeance - and forgiveness - clinical psychologist Gobodo-Madikizela, a member of the South African Truth And Reconciliation Commission (TRC), chose Eugene de Kock as her subject.
He was given the nickname Prime Evil for his activities in charge of the apartheid system's torture and murder squads.
De Kock, serving two life sentences after testifying before the TRC, is here shown to be a complex character. The brutal white supremacist faces the young black woman with a mixture of courtesy and almost avuncular interest.
As a tentative Gobodo-Madikizela (Noma Dunezweni) probes the experiences, behaviour and self-image of this apparent monster, she finds in herself moments of dangerous empathy.
Aware of her subject's emotional pain, at one point she reaches to touch his hand, which he later tells her was his trigger hand.
Matthew Marsh's burly de Kock is a mixture of defeated crusader for a lost ideology and an angry fall guy for a white society only too happy for him to do its dirty work as long as it is all done covertly.
He is too a man desperately trying to find answers to the psychologist's central question: "What makes a man like you?"
Jonathan Munby's intense production moves from lecture hall to barred cell where de Kock, chained by the ankles and dressed in Guantanamo-orange jumpsuit, rehearses past and more recent experiences.
Two key meetings reveal the human being beneath the monster.
There's the moment when he recognises his own dehumanisation in the face of one of his victims as he shoots him.
Later, the widow of another victim weeps for both her husband and his killer as de Kock apologises before the TRC.
There are few answers but many questions in this journey into the paper-thin differences between good and evil.
Forgiveness may be necessary for healing but forgiveness is not forgetting.
Runs until June 15. Box office: (020) 7722-9301.