This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / World War Z
The feral kids. Todd Wainio said that nothing screams louder than a feral who had been shot with a PIE roundnote an incendiary round designed to burn out the brains of zombies. Those kids spent years on their own, having been starved and watching their parents die, only to die a painful death. Even in the post zombie world, they're basically broken shells of children who will never get past their trauma. Sharon, the one who is interviewed in another chapter, is described as being one of the more fortunate cases - she's a grown woman with the mind of a four-year-old, but at least she's retained the capacity for language.
Terry Knox, the last survivor of the astronauts who chose to stay aboard the International Space Station in order to help keep communications satellites in orbit.
"We made our choice, and, I'd like to think, we made a difference in the end. Not bad for the son of an Andamooka opal miner. [Terry Knox died three days after this interview.]
Likewise, the implied death of the U.S. President (implied to be Colin Powell) from the stress of seeing his nation suffer so terribly on his watch, and having to make so many painful, ruthless decisions to see the country through it all. He never even asked about the fate of his extended family in Jamaica.
The mass slaughter, and subsequent extinction, of pretty much every whale species on the planet.
Turtles, too. And pretty much anything else that couldn't elude both zombies' persistence and desperate humans' ingenuity.
Darnell Hackworth's story about hearing the puppies in the pet store next store die of starvation/dehydration, and his immense guilt at not being able to do anything to help them:
"I could hear them from my bedroom window. All day, all night. Just puppies, you know, a couple of weeks old. Scared little babies screaming for their mommies, for anyone, to please come and save them."
He also talks about the 'Eckhart incident', where a dog was luring zombies into a trap but broke his leg, and his handler tried to go get him. When an officer stopped her she shot him in the face but was forcibly restrained. All she could do was listen as the dead ate her 'partner' alive.
A soldier, at work clearing a suburb of zombies, suddenly breaks ranks, and runs into a house. When his friends find him, he's sitting in an armchair dead of a self-inflicted gunshot wound. They look around, and realize from the photos on the wall that...it's his own house. He's come home, and that was the last thing he needed.
Todd Wainio mentions this incident as one of many cases towards the end of the war where soldiers who'd kept themselves together throughout the whole horrible experience started to crack up once the end came into sight. He describes it as something in their heads telling them Hey, buddy, it's cool now. You can let go.
The story of Sharon, the Wild Child with the mind of a four-year-old:
Sharon: [Mrs. Randolph] was Ashley's mommy. Ashley was my friend. I asked her where was Ashley. She started to cry. Mommy told me not to ask her about Ashley and told Mrs. Randolf she was sorry.
Sharon: Arms picked me up and carried me. Carried me into the parking lot. "Run, Sharon, don't stop! Just run, run-run-run!" They pulled her away from me. Her arms let me go. They were big, soft arms.
Brooks' "I love you, Mom" dedication. His mother, Anne Bancroft, had recently died when the book was published.
The IR crews aboard the Ural. These men and women worked as radio operators receiving transmissions from around the world for re-broadcast globally through RadioFree Earth. The transmissions were mostly transmitted over open civilian bands, which were also thick with people desperately screaming for help as they died, and the IR crews' only possible response was to ignore it and search for the official lines to governments, experts and think tanks to distribute accurate, vital information. Every single one of the IR operators committed suicide, beginning with the eighteen-year-old Russian who received the final broadcast from Buenos Aires: a Spanish lullaby being performed by a famous singer. He shot himself right there at the radio.
The final words of the Belgian who was the last of the IR operators:
"You carry those voices with you," he told me one morning. We were standing on the deck, looking in that brown haze, waiting for a sunrise we knew we'd never see. "Those cries will be with me the rest of my life, never resting, never fading, never ceasing their call to join them."
Andre Renard's description of his brother Emile's Heroic Sacrifice during the French effort to clear the catacombs beneath Paris.
They could have withdrawn, blown the tunnel, sealed them in again. One squad against three hundred zombies. One squad... led by my baby brother. His voice was the last thing we heard before their radio went silent. His last words: "On ne passé pas!"
Some captains of evacuation ships would brave the horde of less scrupulous captains and water ghouls to take on more survivors-any survivors; in stark contrast to other captains who wouldn't take Africans, men or old women, or pariahs; and even the ones who fit the pirate's criterion had to give them everything.
In the film, it's Narm to some, but a soldier thinking, "I can't believe it, I'm a Goddamn zeke" is both a tearjerker and Nightmare Fuel. Knowing what's going to happen to you...