Simon kicked the lounge door shut so that he could concentrate, then sat down on the sofa. ‘I’ve been thinking about what you said about serial killers whose targets aren’t individuals,’ he told Moore. ‘Billy’s unusual not so much because he chose pairs as his victims but because he didn’t kill them together. Do you stand by that?’

‘Absolutely. Any serial killer that’s ever gone for groups or pairs as opposed to lone victims has attacked the victim unit together, whether that’s two people, three, five …’

‘And Billy’s not doing that.’ Simon confirmed what he already knew. It helped to say it out loud. ‘He’s eliminating them individually.’

‘And I’m afraid to say that Marion Hopwood’s murder really throws a spanner in the works,’ said Moore. ‘She had no best friend; she wasn’t shot. Billy’s now atypical of a serial killer in more ways than he was when I last spoke to you.’

‘I’m not worried about the not-shot part,’ said Simon. ‘Shoot someone on a crowded hospital ward and you’re going to get caught. Billy doesn’t want to get caught, so it makes sense for him to vary his method.’

‘And the no-best-friend thing?’

Simon chewed the inside of his lip. ‘Yeah. That would be a problem if killing pairs of best friends was what Billy thought he was about. We all assumed it was at first, understandably. Marion Hopwood’s changed that – now we know it’s not about friends.’

‘And we have no clue what it is about.’ Moore sighed.

Simon pitied her. He knew what it was about. It was becoming rather obvious. No one else seemed to see it, though.

‘Let me ask you something,’ he said. ‘Linzi and Rhian were best friends and so were Angela and Josh. But let’s imagine that’s not why Billy wants them dead. He wants them dead because of something they’ve done. In his mind, they all need to be punished.’

‘Okay. Carry on.’

‘I’m going to narrow it down to one pair, though I could use the other just as easily: Linzi and Rhian. Billy wants to kill them both because of something they’ve done. If that thing was, for example, that they got drunk one night and made prank phone calls to Billy’s sister, teasing her and driving her to suicide – if the thing they’d done wrong, in other words, was something they’d done together, then maybe Billy would have murdered them together.’

Simon didn’t generally allow himself to think about the time two of his classmates had spent a whole evening telephonically harassing his mother, and the trauma his family suffered as a result. Kathleen Waterhouse had taken months to recover from the upset, and it was nearly a year before she was prepared to answer her telephone again.