The idea was decent, even if it lacked originality, but the execution and characters were piss-poor.
Some of the worst exposition I've ever seen -- the beginning was one long information dump. The action comes across as one long paragraph after the other of "... and then..." Which didn't only make for flat, lifeless scenes, but made it easy to miss the significance of an event (or miss the event all together) when it was buried in another long, dully uniform sentence.
Writers (and all storytellers) are constantly told to show, not tell. You're supposed to unpack a scene or action so the significance or message comes across clearly to the readers without treating them like they're babies. Unfortunately,
The Killables was one long spoon-fed tell. For example, there was a long series of paragraphs that explained that the men and women were separated at a young again in school, and later in work, to reinforce goodness and purity of thought and action. A comment about missing Raffy when they were separated as young children and another about Evie going to work on the women's floor would've sufficed. The readers aren't completely incompetent. We can figure this stuff out.
To top it off, Evie and Raffy are weak characters who develop very and ineffectively. Evie is pulled and pushed along by other characters for nearly 75% of the book. The few decisions she makes on her own are rash and contribute nothing to the plot. This could work if she were a compelling and empathetic character, aware and bitter of her own lack of action and visibly struggling against it. Instead she's tugged around by either Raffy, Lucas or Linus, who usually trying to teach her how wrong she is in some way, and how she just needs to follow and listen to them.
Raffy has no development at all, and remains sullen and vastly immature. His treatment of Evie borders on manipulative and controlling. He sneaks into her room at night after she tries to break up with them (not just ignoring her specific instructions to leave her be, but also endangering Evies well-being in a dangerous and corrupt system) and pushes her away and ignores her when she admits to him she kissed his brother once -- a kiss, initiated technically, by Lucas, a man nearly twice her age, who had just emotionally manipulated her.
Which leads us to Lucas and this so-called farce of a love triangle. It's obvious the angle for it is there. Lucas clearly has feelings for Evie. But wait, how old is Lucas? If he was fifteen when his father died thirteen years ago, that would make him around 28. Evie is mentioned as similar in age to Raffy, who is 17, if I recall, make her somewhere between 16 - 18. Which means Lucas, a grown man of almost 30, has feelings for a teenage girl. How disturbing is that?
This could've been a very different, meaningful novel if the author had taken a different approach to the subject. Instead it's a weak and offensive one.