We’re in my kitchen, sitting across the table from one another, mugs of coffee in our hands. At one time, I’d have associated a scene like this with ‘A friend pops round for a chat’; now it’s more ‘An enemy whips out a gun and shoots you in the head while you’re putting on the kettle’. I was pleasantly surprised when Simon Waterhouse didn’t, which shows how far my definition of good news has stretched lately.

‘Is Billy a psychological sadist?’

‘Why do you ask that?’ says Waterhouse.

‘Only explanation I can think of.’

‘Meaning?’

‘The four victims before Marion – none of them had terminal cancer, did they? From what I’ve read, they were all relatively young and healthy.’

‘Correct.’ Waterhouse’s way of speaking reminds me of an automated till in a Tesco Express. Please insert payment. Unexpected item in bagging area.

‘My guess is that’s why Billy gave me the first book but still hasn’t killed me. He was planning to – I was first on his list – and then he found out Marion, who was meant to be second for the chop, had terminal cancer. He made a new plan: kill the other four first because there’s no point waiting, and only kill Marion once she’s had a chance to suffer the full extent of her illness.’

‘That’s an interesting theory.’ Waterhouse addresses this unexpected praise to my kitchen window.

‘If you want someone dead, chances are you hate them, right? It might irritate you to think that by murdering them, you could spare them some pain that was coming their way naturally. So you wait as long as possible and kill them just before they’re about to die. You’re a sadist.’

‘And you’re saying originally Billy planned to kill you before Marion, but changed his mind because there was a clock ticking on her and not on you?’

‘My theory’s way sicker than that. He didn’t want me or Marion to miss out on a cancer death. It was gruesome for both of us – more for her, but it’s no fun being a spectator, believe me. And Billy might have assumed I loved Marion deeply, which of course would have made it more painful for me.’

Waterhouse takes a long slurp of his coffee. I wish I hadn’t given him the hideous ‘Hello, is it tea you’re looking for?’ mug.

‘Didn’t you?’

‘Didn’t I what? Oh – love Marion? I don’t know. I expect I did in a way, but … of the feelings I’ve always been aware of having for her, love didn’t figure prominently.’

‘Is that a tactful way of saying you hated her?’

‘There were moments when I hated her. I was always scared of her.’

‘Why?’

‘Long story.’