I think this is the first time I read a "general" fiction book by publisher Tokuma, all the other Tokuma books I ever read where Ghibli-related...
Interesting first experience with Kaji's writings! On the whole, the book follows the very familiar trope of the closed circle situation of a villa during a snowy night, though technically, this is not really a closed circle that is cut off from the world due to the snow: the group of five choose to not leave the villa, because they robbed a bank and have to stay hidden for a while in this house. This is an interesting idea for a closed circle, the "self-inflicted" closed circle (like the sect in Arisugawa's Jooukoku no Shiro), where a (believable) reason has to be given why these people don't just step out of the house and call for help from the police. It works here, I think, partially also because we have Akie, an extra character who is not part of the robbers, so they have even more reasons to not just leave while she has seen them and talked with them. I'd love to see more of these self-chosen closed circle situations actually...
As a closed circle mystery however, Kiyosato Kougen Satsujin Bessou is pretty predictable for about 80% of the book. The cast is pretty small (and the book is short), so the counter of survivors goes down rather quickly, and a lot of the murders are fairly straight-forward. There's often an element of impossibility/implausibility involved with the murders, like when one of them is poisoned, or another is found dead in the hallway even though it doesn't seem anyone could've done it time-wise, but these impossible angles are not really played up to very strongly, and most of the time it seems they shrug it off as 'oh, but perhaps it could've been done anyway'. I think the impossibility angle is kinda hard to achieve, because ultimately, the setting is just one villa, and there's only that much "unobserved" space in a house with 6 people in it, so I guess it would be difficult to really make it feel impossible, but the book tries to feel both claustrophobic, while also trying to sell some deaths as semi-impossible in a setting that's not really suited for it, so that kinda fell flat for me. When you learn the solution to most of these murders, you'll probably just shrug and say 'sure, okay, I guess that works.' (and probably shake your head at the first one). The middle murder might be the most interesting, using a rather unexpected prop in a clever way to create a kind of impossibility, and the hints pointing at the prop can be rather tricky.
But, I have to say, the book really got me at the end. The ending went in a direction I had not expected at all, but in hindsight, I have to admit it was really pretty well telegraphed. Some moments I simply thought there was weird writing, but it turns out they were clues building up to the ending. If the book had not featured this ending, it would have at best an average closed circle mystery (mostly saved by the middle murder), but I have to say this ending does pull the book into the 'oh, I think that was worth reading' territory. The misdirection works really well in this book. The motive of the murderer was not as convincing I think, it might be a 'your mileage may vary' thing, but I think that overall, the book was worth the time I spent on it.
So I did enjoy Kiyosato Kougen Satsujin Bessou as a short read. For a long time, the book develops as a rather predicatable closed circle thriller with on the whole, not really inspired murders, and while the ending may not be the kind of eye-opening conclusion you'll remember for decades like And Then There Were None, I do have to say the book will probably end up a lot better than you'll initially expect based on the middle part of the book. It's a short novel anyway, so it's not like it'll take up much of your time, but it'd be a shame if you'd give up early on it, as I did really like the ending. I'll probably try out more of Kaji's work in the future.