It feels like a whodunnit, making us guess who the killer is, so when the killer turns out to be a exactly the person who we've been given all the clues about. Admittedly when I watched I reacted, much the same as many others, with disappointment that it was too obvious and there was no twist or surprise ending, but perhaps this is exact the point. Before Kate's death there are so many signs and opportunities which people around her could have recognised and if they her had just acted on those or ignored their gut feelings Kate and the children could have been saved.
It turns out that the "whodunnit" we thought we were watching is not that, it a story of domestic abuse, where we the viewers are in denial of the evidence that is being presented to us because it doesn't fit with how we want to view the story, much like the characters are in denial about Kate and Tom's relationship.
It turns out that the "whodunnit" we thought we were watching is not that, it a story of domestic abuse, where we the viewers are in denial of the evidence that is being presented to us because it doesn't fit with how we want to view the story, much like the characters are in denial about Kate and Tom's relationship.