These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Moral Event Horizon: Linnet's murder is one, especially considering that Simon strung her along, pretending to love her as she did him, and then shot her in her sleep.
Tear Jerker: Simon and Jackie were very much in love, and could have been happy together but for Simon's greed which ultimately brought about both their deaths.
Values Dissonance: Poirot tells Linnet that there's nothing the police can do about Jackie following her and Simon around all the time because, as long as she doesn't actually threaten them, she isn't doing anything illegal. Stalking and harassment laws obviously hadn't been invented yet.