Considering the 'success' of the disappearance of Mrs Murphy's body, it is not unreasonable to factor in that he may have prepared the place to hide the body long before . Not necessarily Mrs Murphy's body, per se , but any woman's body that became awkward for him, that told an incriminating tale about him.
Which presupposes that the premeditation element may have been a long time incubating. The factor of surprise being whichever victim he assigned as 'the one.'
Nobody in particular, but whoever crossed his path, he had everything ready, perhaps... tools, hiding place, transport, change of clothing, all the details ( except, we can presume, one major , colossal detail he forgot, hence 'hours of CCTV ).. not an impulse, not a whim, but a long imagined fantasy, with regularly updated details in his mind...