All is not quite right with Belko, a former juvenile offender that Rosie's father Stan (Brent Sexton) hired to work at his moving company and took in like an adopted son (or creepy uncle). Belko's got no life of his own beyond a troubled situation at home with his overbearing, promiscuous mother. He's latched onto the Larsens to the point where he has his own set of keys and sometimes lets himself into their house when no one else is around -- which is what he did the night that Rosie was killed. Belko may love the family a little too much, and his capacity for violence is demonstrated not just in his heading off to shoot Darren in the finale but in his earlier offering to Stan to "take care of" the suspect, a reference to their tough pasts. Plus, the collage of photos of the Larsens, including Rosie, on the ceiling above his bed suggests a creepy fixation that could have gone wrong.
Rosie's hip young high school teacher likes to pay special attention to the bright girls in his class, and he not only wrote Rosie long, personal letters of encouragement and praise, he also seemed to be a closer to her than is typical. Then there's his very pregnant wife Amber (Ashley Johnson), who was also once his student, and who recounted to the detectives details about how they got together that sound uncomfortably like his relationship with Rosie. Investigating Bennet lead the series down a rabbit hole of terrorism scares and female circumcision in the Somali refugee community, but though the girl he was involved in hiding turned out to be very much alive and on the run from a traditional marriage, he still could have killed Rosie -- there are suggestions he's still hiding something to do with her, and he has a lot to lose. The brutal beating he took from her father when the man was convinced he was the killer was awful, but also didn't necessarily clear him.
The idealistic mayoral candidate looked to be fully pinned down at the end of the first season, until a photo placing him in the car in which Rosie was found turned out to be a fake. But that doesn't mean Darren didn't do it, just that there's no proof. The principled politician has turned out to have one hell of a dark side, though as his former lover and campaign manager explained, "He's good at compartmentalizing his grief, his emotions -- shutting off. I think that's what makes him a great leader. He doesn't let others see his wounds." Does it also make him a great killer? His obsession with women (some paid) who look like his dead wife implies a deeper instability, and he's certainly got the smarts to hide his tracks well. Now that he's already been arrested for the crime, access to him or his campaign will surely become almost impossible for Sarah.