In Behind Her Eyes, Netflix was right to cut a key horrific scene from the book despite the fact that it demonstrates just how evil Adele (who's actually Rob) is. The book's scene involves the brutal murder of Charlie, the feline companion of Marianne, a platonic friend of Adele's husband David. In the book, Adele murders the cat in front of Marianne in a deranged attempt to keep her away from David.
In Behind Her Eyes' season finale, though, Netflix lets the cat live. When Marianne tells Louise the story of her confrontation with Adele, she recounts getting home to find Charlie missing. She searches for the cat throughout her house and finds it ransacked, the walls vandalized with what seems like blood. Both Marianne and the audience can't help but imagine it's the cat's blood, and fans of the book assume that they know what's coming. Just as the tension reaches its height, however, Charlie innocently jumps into the scene, giving a startle and a sigh of relief for Marianne as well as the audience.
Although the cat's murder scene is a perfect representation of the lengths to which Adele will go to keep her husband, Netflix was right to cut it for at least two reasons. The first reason is that the murder depicted in the book is extremely graphic and cruel, which would most likely take casual viewers by surprise, but not in a good way. Adele confronts Marianne for suggesting that David leave her, and to drive home the point, she stomps on Charlie's head with her high-heeled shoe. This type of brutality might be expected from other genres, but not from a supernatural psychological thriller like Behind Her Eyes. Additionally, for better or worse, audiences often view cruelty to animals as even more horrendous than cruelty to humans.
The second reason Netflix was right to cut the book's cat-murder scene is that because it is such a vivid and memorable element of the story, fans of the book would be waiting for it, which would ultimately rob them of some of the TV show's suspense. Subverting fans' expectations by letting Charlie continue living his best cat life is an effective way of playing with the audience's emotions without having to go to the same gruesome lengths as the book. As a matter of fact, there are several omissions that the TV show makes that fans of the book would have been expecting. Many of these scenes, like the cat's murder, contain potentially offensive scenes that would possibly alienate viewers.
One of these scenes from the book is particularly disturbing and would possibly enflame political opinions about the show, which is relatively far from being political. When Rob initially takes over Adele's body, Adele is pregnant. Rob is horrified about the thought of giving birth, and he aborts the pregnancy. Some viewers might judge the entire show based on Rob's decision, despite his being clearly evil and his actions being condemned by the series itself. Another omission along these lines is that the book explicitly states that Rob, now in Louise's body, intends to kill Louise's son, Adam, which can only be inferred in Behind Her Eyes.
By all accounts, Behind Her Eyes is a faithful adaptation of the book, but that doesn't mean that certain fans won't be disappointed by the omissions. That said, these omissions arguably enable the TV show to reach a wider audience, and in turn, they may even inspire viewers to read the novel. If the end result of sparing the life of Marianne's cat is that more people pick up the book that inspired Netflix's Behind Her Eyes, then it wouldn't make much sense to be disappointed.