In Marcus's house, following the explosion of anger which left Clive Dollis dead, the rational Rufus realised it'd not been sensible to slaughter the stranger. He'd have been the key to finding Marcus!
Rufus's rationality was the machine's contribution. In that sense, the machine had been brilliant, excepting its inability to tame Rufus's impetuosity. The boy was a split personality, but both elements were starting to feel constrained by the need to stay connected to the machine; like an adolescent starting to challenge its father.
But father had been canny. Deep down, embedded in Rufus's cortex was a dependency drug.