Tear Jerker / Red Mars Trilogy

• Although John Boone's death at the beginning of Red Mars is clearly not good, it probably doesn't have a huge emotional impact given that you don't know anything about him. After seeing that he was, though flawed, a benevolent person with an incredible ability for bringing people together, it becomes clear what a tragedy it was that he was killed. And his POV section ends just before Nicosia.
• The death of Arkady Bogdanov in 2061, which is just the start of many horrors. On the start of a promising day of rebellion and revolutionizing society, the transnationals pump his rebel city's tent full of oxygen and then set it on fire, immolating everyone within. And this becomes a standard tactic for them.
• Nadia's reaction to discovering said immolated tent settlement. She promptly enters Heroic Safe Mode.
• Against all odds, Frank Chalmers dying in a flood after getting the car unstuck. Despite being a singularly unpleasant person who arranged the assassination of his old friend and the most conventionally heroic person in the story, after getting to know him through his own point-of-view makes hating him much more complicated.
• The above makes Ann go It's All My Fault and attempt to kill herself via exposure, since she got distracted while driving. Simon stops her, but it doesn't make her depression and guilt go away—instead, she remains a borderline Death Seeker well into Green Mars. It's exacerbated by Simon dying of leukemia.