This is based on opinion. Please don't list it on a work's trope example list.
Tearjerker / Respect A Woman
The Not Immune Scenario. My GOD, the Not Immune Scenario! Especially Smoker's and Jockey's.
Basically, the premise is that one of the four (there's one for each of them) is Not Immune to the Green Flu. They realize what's happening, and are forced to choose-do they kill them before they can turn, lock them up somewhere, or just let them get as far away from the group as possible? No matter what, they're going to die.
Then there's the Sacrifice Scenario, which is the same as the one in Left 4 Dead. Someone needs to sacrifice themselves to save the others. Again, we get to see how things play out if each one decided to give their life for the others.
Jockey's last words are particularly heart-wrenching.
Jockey: "'Itíll be okay guys! Donít worry! Iíll see you all again soon! Iím gonna go find Travis and David and the others, and Iíll tell them where you are so they can find you! Weíll all be together-'''
The comic that begins the Infected universe. Hunter gets bit by one of her parkour friends as the infection begins, and rushes to Smoker. She feels herself beginning to turn, tries to fight it, fails, and attacks Smoker. She leaves her barely alive and infected in a bathtub.
Jockey gets gifts for her friend Travis, Hunter's parkour buddies, and Charger's husband David for Christmas. They're all dead, but Jockey continues to act like they're all alive and well, and twists the knife deeper and deeper in Charger.
Charger finds some dead livestock and starts musing on on how the zombie apocalypse has devastated the already-fragile ecosystem.
Smoker turns into a smoker curled up in a fetal position on her bathtub floor, feverishly begging for Hunter to help her.
David comes back as a zombie. The whole thingís just a great big mess. Jockey ends up being the one to shoot David and is desperately pretending that sheís doing anything but that, and doing a pretty poor job of it. She lapses into a Heroic BSOD immediately afterwards.
Charger has dreams where Davidís alive again. When she wakes up and heís not her face is heartbreaking.
Jockey staying with her friendís corpses and refusing to abandon them.
In the Puella Magi crossover, Charger tries to kill the others to stop them from becoming witches. She kills Jockey and Hunter before Smoker kills her. Then Smoker's all alone...
Jockey and the other survivors finding two more of Jockeyís friends dead: Lorelei and Lenora.
Mousey messes up (exactly what happens is left unclear) and Charger and Smoker go looking for her. When they find her, Smoker has a relapse, and attacks Mousey, triggering Chargerís relapse and further traumatizing poor Mousey, who leaves in tears.