The description of the killer's motive. His father killed his mother for having an affair while the boy was just a couple of rooms away and has spent the past two decades acting like nothing happened. If not for the fact that he ends up killing two other people during his Best Served Cold attempts at revenge and tries to kill Wolfe, he would be an Anti-Villain through and through.
While the father is a thoroughly unpleasant person, his realization that his son wants him dead can still be sad.
Helen Frost's sense of guilt at inadvertently obstructing the investigation when this contributes to the death of her Parental Substitute. Finding out that he was her actual father and that the killer is the woman who raised her doesn't help.
Just seeing how badly hung up Recurring Character Marko Vukcic is over his callous ex-wife, with Wolfe noting that he was a different, happier man before his marriage. Then, in the penultimate chapter, Marko learns that she conspired to frame him for the murder of her new husband.
The death of Phoebe Gunther, who engages in a Battle of Wits with Wolfe and Archie that extends beyond her death and leaves them missing her presence more than they'll admit.
Potential client Cynthia Brown laments about how she's been too cowardly to give the police information about the murder of her best friend. Then, when she's working up the nerve to (and plans to give up her life as a Con Artist), she gets killed.
The entire situation of Peter Hays, who is repeatedly Wrongfully Accused of crimes. The first time, his own father is the accuser; the second time, he erroneously thinks the woman he loved did it (the victim is her abusive husband) and resigns himself to Taking the Heat for her.
The He Knows Too Much motivated murders of Joan Wellman and Rachel Abrams, both of whom have suitors, loving families, friends, jobs they liked, and no enemies. The scene where Joan's father and Rachel's mother talk about them to appeal to witnesses for information is just heartbreaking.
Mr. Wellman: [I]f you gave me a pad of paper and a pencil and asked me to put down all the different things I can remember about Joan, I'll bet there would be ten thousand different things, more than that—things she did and things she said, times she was like this and times she was like that...I was wondering if I deserved what happened because I was too proud of her. But I wasn't. I thought about it this way, I thought there had been lots of times she did something wrong, like when she was little and told lies, and even after she grew up and did things I didn't approve of, but I asked myself, can I point to a single thing she ever did and honestly say I wish she hadn't done that? And I couldn't.
Mrs. Abrams: If she got some dollars ahead I would say, "Buy yourself a pretty dress or take a little trip," and she would laugh and say, "I'm a working girl, Mamma." She called me Mamma, but Nancy and Deborah called me Mom, and that's the whole difference right there...I try to see why that happened, and I can't. I don't care how much explaining I get, I don't think I can ever see why any man had to kill my Rachel, because I know so well about her. I know there's not a man or woman anywhere that could stand up and say, "Rachel Abrams did a bad thing to me."
Both the second and third murder victim are people who've interacted with Archie at length and charmed him and the readers to an extent. His It's All My Fault feelings about failing to save them is equally depressing.
Wolfe gets hired by a ten-year-old boy, who has uncovered information about a potential murder. Because he has that information, the killer runs him over shortly afterward, and he dies while being taken to the hospital, with his mother at his side.
In The Zero Clue, Susan Maturo talking about how her fiancée and several friends died in a hospital bombing with a triple digit casualty rate. The reveal of the petty motive behind the bombing is even worse.
The death of a blackmailer normally isn't too sad, but this blackmailer's kindly parents appear and receive a good amount of characterization well before her murder, allowing the reader to feel for their loss even before seeing their reaction to her death. The blackmailer's youth and history of Comically Small Bribe demands ($5.00 a month and theater tickets in one case) can also evoke sympathy.
