When in 1994 Robert David MacDonald staged Into That Darkness, Gitta Sereny's study of Nazi extermination camp commandant Franz Stangl, it was twenty years since the Austrian born writer's book was first published.

The book itself had resulted from some sixty hours of interviews with Stangl, in which he eventually admitted his guilt before suffering a heart attack nineteen hours later.

MacDonald's production was staged under the title In Quest of Conscience at the Citizens Theatre in Glasgow, where with Giles Havergal and Philip Prowse he was the theatre's co-artistic director. The playwright, translator, adaptor and international polymath himself played Stangl opposite Roberta Taylor as Sereny.

Another twenty years on, and the Citz has restored the book's original title for a new look at MacDonald's version of Sereny's book which opens this week in a production by Gareth Nicholls. This time out Blythe Duff takes on the role of Sereny, with Cliff Burnett as Stangl.

Sereny, who died in 2012 aged ninety-one, went to see the original production of MacDonald's play with her entire family, including her photographer husband Don Honeyman and their two children. Sereny's daughter Mandy Honeyman will return to the Citz with her brother and sister-in-law to see Nicholls' new take on things.

"We want to go and see it again," says Honeyman, "and remember the book and remember our mother. I remember going up to Glasgow before with my mum and dad, and I think my mother found it quite odd being represented onstage, though for me it all felt once removed."

Into That Darkness: from Mercy Killing to Mass Murder, a study of Franz Stangl, the Commandant of Treblinka was Sereny's second book, following the equally controversial The Case of Mary Bell two years earlier. That book focused on the eleven year old girl who was convicted for killing two four year old boys in 1968, and, as with Into That Darkness, attempted to go beyond received notions of evil to find the humanity beyond her subject's actions. With Stangl, however, this wasn't easy, and the experience left their mark on Sereny.

"I think it was incredibly hard for her," Honeyman remembers. "I was a young teenager, and my dad looked after me and my brother pretty well, but out of all of mum's books I think she found this one the most difficult. Stangl was quite an ordinary, boring man, but the conclusion she came to was that there were no redeeming features about him.

"My mother was having nightmares while she was doing the book, and afterwards she was quite ill. I was protected from all of that to a degree, although I understood everything that was going on, but we kind of lived around it for three years."

Once published, Into The Darkness entered into the public domain to such a degree that even satirical magazine Private Eye picked up on it.

"That was the funniest thing," Honeyman remembers. "They did a cartoon at a football match where they announced they would play my mother's voice through a megaphone to stop anyone getting out of line. My father found it very amusing, though I think she was slightly less amused."

Into That Darkness is the biggest project to date for the Citz's Main Stage Director in Residence Gareth Nicholls, who assisted the theatre's artistic director Dominic Hill on his production of Hamlet, which featured Taylor as Gertrude.

"It's a complex piece," Nicholls says of Into That Darkness, "especially for Blythe and Cliff in terms of the psychology of the characters. The people they're playing were real people whose actions affected a lot of people, so there's that to deal with as well.

For me one of the great things about the book and the play is that they don't shy away from difficult questions that aren't black and white. It questions notions of responsibility through action or inaction, and notions of truth and identity. Gitta Sereny brings together so many different accents to the piece that either confirm or contradict what Stangl says. There are notions there too about otherness, and Sereny points out that the people doing these terrible things are people like you or me."

Like Sereny, MacDonald, a scholar of German literature, returned to the Holocaust several times throughout his career. This was the case both in translations of German writer Rolf Hochhuth's plays, The Representative and Judith, as well as his own play, Summit Conference. Where The Representative, first seen in the UK in 1963 and at the Citz in 1986, suggested papal indifference to the Holocaust, Summit Conference imagined a meeting in 1941 between the mistresses of Hitler and Mussolini.

Interestingly, Hochhuth was a friend of author and Holocaust denier David Irving, and drew on some of Irving's early work for Soldiers, another play translated by MacDonald. Soldiers had its production at the National Theatre withdrawn in 1967 following a libel suit issued by the surviving pilot implicated in the play as being part of a conspiracy to kill the Polish Prime Minister in a 1943 plane crash.

Irving himself initiated a libel case against Sereny following her assertions in a newspaper that Irving had deliberately falsified the historical record in an attempt to rehabilitate the Nazis. The case never made it to court.

As for Into That Darkness, it may be seventy years since the end of the Second World War, but Sereny's dissection of Stangl remains as pertinent as ever.

"Stangl committed horrible crimes," says Nicholls, "but there's a temptation to think he's somehow different from us. It's dangerous to think that, so instead of ignoring the reasons behind what made someone do what they did and locking them away, to stop them happening again you have to try and understand why these things occurred."

Honeyman agrees.

"Stangl did some of the most terrible things anyone has ever done," she says, "and people keep doing things like that in one form or another. One should always go back to re-review these things in the context of where we are now, and that's why it's good the play is being done again. People have to keep on talking about what happened around the Holocaust to realise that ordinary people can do terrible things, but nobody is born evil."

Into That Darkness, Citizens Theatre, Glasgow, May 18-30.

www.citz.co.uk