The killing of the well-liked and respected Clutter family was a complete mystery. According to KBI Agent Nye, the general consensus in town was that, "Of all the people in the world, the Clutters were the least likely to be murdered." (2.19)
The striking thing about Herb Clutter as he's described in this book is his consistency as a character. Nothing he does is confusing or surprising; his motivations are clear and consistent. He's a "man's man" (1.6), a strict parent, proud of his kids, raising them to be hard-working and responsible; a devoted husband to a troubled wife; a hard-working provider for his family. He's "the community's most widely-known citizen […]" (1.6). He's fair and generous in his dealings, unpretentious in his social interactions. He doesn't drink (not even coffee) or smoke. He got an education and worked hard to be a success. Not only all that—he cooks!
The only bad thing we hear about him sounds like a bit of envy from some of the townsfolk—that he always got all the good jobs and is too busy rushing around on all the town committees and boards that he chairs. He's not the most emotional guy in the world, but the book does take place in the 1950s, and that was wasn't unusual for the traditional male of the time. He sues a guy who crashes a plane into his orchard, but that's just business. Even his killer thinks he was a very nice man.
So how does Herb attain this perfection? Well, despite being a decent guy from all reports, he's dead by about page 25. We don't get much time to know him except from the perspective of his friends. Had he been a living, breathing character throughout the book, we would have likely seen more complexity or change. We don't get to see any development of his frustration with his wife's illness, or philandering because he doesn't have a sex life anymore (a love triangle murder motive is dismissed out of hand), or loud arguments with his kids, or physical illness—he's a static character.