There seem multiple reasons advanced above, which makes one wonder about the real motive, that of granting him an alternative weapon of execution: five bullets for five victims and a clean kill.
Not in strict chronological order, but the month preceding the murders offers an insight into his mind: the prescient knowledge relayed to Ann that they would in the near future be running the caravan park together, his boasting that he could convince anyone of anything, the annual allowance June was about to grant her daughter, his admission to Andrew Bishop on the occasion of his stag night that he may be mentally ill akin to his sister.
The giveaway was the birthday card sent to Ann in late September 1984, by which time the murder plan was already germinating in his mind, knowing that he had to endeavour to keep his cousin onside. The remark to Colin at the Saturday party that Nicholas and Daniel were "a millstone round his neck" was an attempt at justification of the diablerie he was about to perpetrate. He had to be in control, he had achieved de facto control at the time of the murders and he still attempts vindication from a prison cell by maintaining innocence, thereby keeping a modicum of control over adherents to his cause, whilst the shocking truth for all his deceitful stories was as he told Julie: "For the way they have treated me my parents have forfeited the right to live."
