For my money, good crime writing gets under the skin of real people, based in the real world. At its best, it gets deeper under that skin than other genres. But there's the temptation to flirt with the grotesque - to reach new heights (or depths) of depravity - to shock the reader in ways they haven't been shocked before. Taken to an extreme, this can fracture the connection between writer and reader; it takes a highly skilled writer to hold our attention while turning our stomachs.
I'd rate Thomas Harris' Silence of the Lambs above just about any other in this regard, although it's interesting that what engaged most readers - and the author himself - was the human relationships at the heart of the story and not its serial killer with the dodgy personal habits. I'd also point out how much more I enjoyed Mo Hayder's recent novels to her earlier ones, where the shocks came thick and fast but the characters cried out for a deeper exploration - which, in the case of Jack Caffrey, she served up in the later books, in spades.
Crime writing which relies heavily on shock value, on excessive brutality or grotesquerie, usually strays into farce. I'm not saying brutality isn't a part of reality, of course it is. But if the reader can detect the sweat, blood and tears of the writer in the effort, it severs the link, takes us out of the story into boggling at the craft beneath.
A good friend told me recently she'd given up on 'hardcore crime' after reading a particularly bloody and bizarre passage which had the opposite effect to that intended by its hard-working author: it made her laugh. Not because it was funny, but because she could sense the effort which'd gone into its nastiness. 'This,' she could clearly hear the writer thinking, 'is the stuff to give 'em!'
Yep, I had a similar reaction to Mo Hayder's Pig Island. But by the time she was writing Skin she'd put the shocks (and the laughs) in their place; Skin is a terrific read, exciting and haunting, funny and tragic. Balance is everything, and Hayder's got the measure of this now.
Natasha Cooper wrote a great piece around this topic, a couple of years ago, but her words have stood the test of time. Read her advice on the Dangers of Crime Writing here.