If you want proof of NM's fundamental cruelty, you have only to witness her treatment of that most loathsome of objects -- a person with a head cold.
Dorothy Sayers could be equally unsparing, but in general, her impartial catalogs of human frailty make her readers feel compassion, rather than embarrassed contempt. Her pitiless recitation of weaknesses -- petty flaws, awkward moments, hypocrisies, gaffes, people exposing themselves so dreadfully -- has the affect of making those people seem worth pitying. We are fragile, we are human, so we deserve protection and justice. Sayers' flawed humans deserve Peter Wimsey's best efforts because of their flaws, not despite them.
What NM thought her flawed humans deserve is a little less clear. What may shift the dynamic in NM's novels is that she never (that I've noticed) leaves her main characters exposed. Troy and Alleyn are so contained that everyone else seems even more horribly naked in comparison. Even late-Sayers' Wimsey is occasionally ridiculous, but the only thing that makes Troy and Alleyn squirm is vicarious embarrassment. Or, I suppose, ominous forebodings.
This is another mystery where NM decided to tackle race relations. I would rather she hadn't since I wouldn't say she did it well, although the book does become an interesting historical artifact in this respect. It's not as horrible as Sayers' blatant prejudice against the Scots, but it's also not as cheerful as Sayers' (also problematic) treatment of Jews. Sayers gave her characters the attitudes she gave them and doesn't judge those attitudes one way or the other. Which is unfortunate. NM gave her characters the attitudes she gave them and then (in the case of Troy and Alleyn) congratulates them for their enlightened point of view. Which actually makes me feel queasier, since that point of view is, essentially, that there's no reason to feel prejudice toward a Negro once he has successfully made himself over into an Englishman. It is culture that matters, not skin color. I find it easier to ignore in books I otherwise like over the top, wildly dated racism than I do partially-evolved, over-intellectualized racism.
NM also anatomized the state of race-consciousness in the other characters. What were meant to be subtle slurs have no subtly and that ineptitude renders the characters both bigoted AND gross. And what were meant to be performances of progressiveness aren't performed with conviction or comfort, leaving those characters foolish, mouthing ideas they don't fully understand.
Then there's a whole hodgepodge of other stuff -- art theft, tacky Americans, the signs of the zodiac, the inevitable limitations of the audience of students to whom one is speaking, Romanticism, the annoyance presented by motorbikes, and a winding river.
Ultimately, like so many mysteries, this is probably a book about change -- a vanishing old world, and an awkward, damp and scrawny new world that's still staggering around fragments of shell. I guess there's something about change that makes people feel murderous.