Can we take McEwan's Nutshell seriously?

Stop me if I'm wrong but seriously how can you take seriously the behavior of the characters in Ian McEwan's Nutshell? - and I'm not even talking about the governing conceit: that a fetus in utero could narrate an entire novel based on what he overhears from within the womb - and based on detailed knowledge not only of the English language but of all customs, cultures, arcana - OK I get it we're not expected to take the narrator as a realistic possibility, he's just a device to convey to us a narrative entirely depending on someone's overhearing the murder plot (unless if were told by an omniscient narrator, how boring), but what's the point if the overheard plot is preposterous? Yes, it's based on Hamlet, how clever, with clues and snippets of quotes cleverly strewn about. Yet does the Hamlet plot translate in the 21st century? Maybe 500 years ago with a kingdom at stake a jealous man would poison his brother to take over the kingdom. But there's not even a plausible motive in this plot: Gertrude dumps her husband, John, to get together w/ his brother, Claude - but as it turns out John is fine w/ that, he has a new romance of his own, and all he wants is to take back possession of their nice house - which really shouldn't be much of an issue because brother Claude is far more well to do. So for this they actually kill a man - not in rotten Denmark but in contemporary London? These characters would have to be seriously deranged if not demonic to do so, but, no, they seem like pretty ordinary Londoners - Claude in particular is quite hilariously played as a dullard, the world's worst conversationalist - so what causes them to carry through with this most drastic and dastardly of acts? They don't think they'll get caught? Jailed? Hanged? Why not? Just what's going on here, beneath the clever witticisms that sparkle on the surface?