What's frustrating about Nutshell

What's frustrating about Ian McEwan's novel Nutshell is that McEwan can be so good at times - several riffs, paragraphs, pages of the narration that I pause to mark with a note or just an exclamation point - litany of all things wrong with the planet in a page or less, speculation on what the world will be like, or not, at the end of the century, others - wish I could recall them better, maybe wasn't as attentive last night as I should have been, will look some up when I get back to the novel - but the point is all of these great passages are to what end? A totally bizarre narrative conceit and a murder plot that doesn't pass the sniff test: I know the world is filled w/ lurid crimes, or at least the nightly news is so filled, but there's nothing for 2 seconds probable about this couple committing this coldly calculated and simply pointless murder - and even if they well so what? Do we care about the characters for a second? Do we believe in the narrator for a second? All this would be less frustrating if this were just a plain old bad book. It's not. There are many moments of brilliance. But in the end  - well, I'm not quite there yet - this book reads like a pot pourri of brilliant ideas and observations in search of a novel.