So maybe Jin-mook knew who the killer was, so they gave him the fishing line so he wouldn't reveal their identity. Chief NSB visited the cell that night so he witnessed the incident. Perhaps SB confronted them (explained why he was waiting in the parking lot. Maybe it was supposed to be a "talk-it-out" kind of deal but both of them ended up having a plan to kill each other). That indicated he knew them on a personal level (that made them one of the Manyang folks). So Yae-jin saw SB->SB saw the killer killing JM-> killer killed SB.
Over the top and over combined. To make it more interesting, they make every character at some point looking guilty…
DS in a way showed a lack of ability to express his emotions (held back tears, acted erratic, ect), so it's easy to understand his crazy fits of laughter. It’s actually not out of place at all. He only did that when either someone was accusing him of murder or something unfair happened to the people he cared about, putting him under a lot of distress which he kept to himself (calmly attending dinner session). The reason why it's unnatural is because that's the point. It's not a normal way to express anger/sadness so you can really see how this character's very close to going insane. Personally I think it's a nice way to help us understand how other characters viewed LDS. His laughter/smile was super unnerving and that's why they called him a nutcase. Also, I personally love that they make the whole town shady as hell. I understand some characters' behaviors may come off as too intense or illogical, but let's think of it in another way: It's a tight-knitted community where a person close to them died. Most of them were all involved in a murder that related to the disaster 20 years ago. Each has their own secrets, and now an unfriendly outsider came in to poke his nose around. I can't see why they don't appear guilty at some point trying to hide their ulterior motive, most likely to protect themselves/others.
Also, I personally love that they make the whole town shady as hell. I understand some characters' behaviors may come off as too intense or illogical, but let's think of it in another way: It's a tight-knitted community where a person close to them died. Most of them were all involved in a murder that related to the disaster 20 years ago. Each has their own secrets, and now an unfriendly outsider came in to poke his nose around. I can't see why they don't appear guilty at some point trying to hide their ulterior motive, most likely to protect themselves/others.