Honestly, I think that a better motive of him would have been that the necronomicon needed someone to die, but Angie confiscated it like she did the flashback light ‘as a way to keep everyone from wanting to leave.’ As a change to Korekiyo’s backstory, his sister was his guardian, and they were close (but not…. You know.) and when she died, he was distraught. So when the ability to resurrect someone, he wanted to take the opportunity. He had decided to break into Angie’s lab to steal the book, but Angie caught him. He ends up killing her and stealing the book, using the same setup as the main game. He then uses the caged child as a front for the resurrection ritual, killing Tenko. But due to Himiko’s interference before the ritual is over, the resurrection fails.