Your inquiry into the subject of killing obviously began as a personal curiosity, spawned by the grisly discovery of a souvenired soldier’s head from Gallipoli and the questions that sparked about the nature of violence. When did you realise it would become a book?

Originally, I wanted to do a whole project about the mummified skull from Gallipoli. It was such a shocking artefact – not simply because it was a bullet-ridden body part, taken from the trenches, but because it had been stored in a velvet lined display cabinet, like a precious collectable.

I thought I could try to find out what had happened and use that story to discuss the Great War, a topic that’s long fascinated me. That particular idea collapsed because there simply weren’t enough clues – at least not that I could find – as to how the Turkish soldier’s head arrived in Australia. So for a while, I abandoned the whole thing. I only took it up again after reading a news article about how US soldiers in Iraq were collecting photos of corpses. It struck me that this was probably the same phenomenon that led to the souveniring of a head from the battlefield of Gallipoli, all those years ago. So I started wondering what war in general – and killing in particular – did to soldiers and to society. And then I spent a long time thinking about the different ways you might investigate such a topic.

The book is written in such a way that the reader accompanies you on your search for answers, rather than being presented with your findings. I thought this made the book particularly engaging, and encouraged the reader to draw their own conclusions and interrogate their own beliefs. Why did you decide to write it in this way? Was it a conscious choice?

It was partly forced upon me, in that very early it became apparent that getting access to people and material would be difficult. So I wanted to foreground the process I took and the difficulties that I faced, to talk about the information I couldn’t get as much as that which I found. But it also seemed appropriate in that most of the time I was genuinely conflicted about the material. In the (slim) literature about killing, you can find people describing combat as the worst moment of their lives – then a minute later, discussing how nothing they’ve done since has been as exhilarating as the few minutes they were in battle. I kept meeting different people who argued different things, and I wanted the reader to share that experience of the perspective shifting. Moreover, it’s an area in which it’s difficult not to become emotional. I spent some time with a former executioner in Virginia, a guy called Jerry, and he was one of the loveliest people I met when I was in the USA. That was quite a confusing experience that took me well out of my comfort zone, and hopefully the book conveys that.

The industrialisation of killing (‘McDonaldised killing’) is a running theme in the book – a separation of the act of killing into a series of tasks, or a collective act, whether in the abattoir, on death row or in the army. This enables those involved in killing to concentrate on the task, rather than on the bigger picture of what they’re doing. What effect does this have on the people involved?

In the short term, it makes a distasteful or horrible task easier. On death row, the tie-down teams rehearse again and again the particular role they will play in an execution. One guard might simply be responsible for strapping the prisoner’s leg onto the electric chair. If he concentrates on doing that, he can lose all context for what’s really happening – he’s not killing someone, he can tell himself, he’s just strapping a leg. In the longer term, however, there seems some evidence that it can have the opposite effect. Because the killing takes place without any emotional affect, it’s difficult for the perpetrator to come to terms with what’s happened. Some executioners then feel devastated precisely because they don’t feel anything.

There’s some analogies with what takes place in a modern abattoir (basically, a factory for killing animals) and, perhaps more controversially, in the way soldiers are trained for combat.

One of the things I really like about this book is the way that you interrogate your own beliefs on the subject and the fact that you share your doubts and changes of position with the reader. What were the most confronting moments for you in researching this book, in terms of challenging your beliefs?

When I was young I was a vegetarian, but I haven’t been for years. I really don’t know any more what I think about the industrial killing of animals. Certainly, it’s hard to visit an abattoir and not think you’re watching something very wrong taking place.

I also found it confronting how nice and now normal most of the people I interviewed were. That sounds fatuous (of course they were normal!) but it’s much easier to think about killers as monsters than to realise that they’re ordinary human beings. So, it was a challenging experience hearing a former US sniper talk about the men he’d shot, even as you’re thinking that he seemed just like any kid you’d meet in Carlton.

One of your findings was that once the process of killing becomes familiar, ‘the participants worried more about efficiency than anything else’. That was also your own experience while helping out the Queensland roo shooter you interviewed. Did that finding surprise you? What do you think it says about human nature and violence?

Humans are social animals and so I suppose it shouldn’t seem strange that social approval matters so much. But, yes, it did surprise me how much I wanted the approval of the guy who took me roo shooting, even though it’s not something I’ve ever done before and I can’t imagine ever doing it again. In the book, I quote Siegfried Sassoon, discussing some particularly awful event in World War I and mentioning that, more than anything else, he was worried about making a fool of himself. It does seem to be a common experience.

Execution expert Fred Leuchter observed of the workers who administer the death penalty: ‘Society has dirty jobs that have to be done.’ What do you think of this comment? Does it also pertain to the abattoir workers and military? Do you think we need to be more assiduous about measuring the cost of these ‘dirty jobs’ against their gain?

Ultimately, in all the different forms of violence, the responsibility lies much more with those who make the dirty jobs necessary rather than those who carry them out. The book is not supposed to be an attack on soldiers, abattoir workers or prison guards. As I said, most of the people I interviewed were actually very pleasant.

But somebody sends the kids to war; somebody sentences an inmate to death; somebody wants a cattle truck processed – and it’s that somebody we need to think about. The breakdown of killing into isolated acts affects those personally involved in the act, but it also helps further separate those end of the chain of command from the process they set running. At the most obvious level, you can, for example, launch an invasion of Iraq and never see the human consequences of that decision. The soldiers don’t have the same luxury.

Were there any particular books or authors who influenced you in writing this book? If so, what (or who) were they?

I found Paul Fussell’s book The Great War and Modern Memory a fantastic resource. It’s one of those books that completely changes the way you think about a subject. Basically, he shows how the war structured a whole range of things that we now just take for granted. I also became completely obsessed with Sassoon: less as a war writer, actually, than as a chronicler of the world destroyed by 1914. Books like Memoirs of a Fox Hunting Man and The Old Century have a tremendous power because, even though they’re set in the pre-war era, they’re so obviously haunted by what comes next.