This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Eight Mercenaries and a Toddler
The RED Spy's backstory. And "There's no medical assistance needed" is quite a powerful one, too.
Little Scout getting shot and dying in Spy's arms.
Spy raised a trembling hand to Scout's back, rubbing it in small circles. "Don't cry, Scout…" he whispered, "I'm right here." He buried his face in Scout's hair. The little hands holding onto him slowly loosened their grip.
The RED team's reactions to Scout being kidnapped by the BLU's
I'll Be Home for the Holidays:
Chapter 15. Sniper is so filled with shame and revulsion when his family finds out what he does for a living he just turns from their faces and flees.
Pretty much all of Chapter 17.
Spy's entire confession to Sniper in Chapter 23.
Over the course of most of the story Medic is alone at the base, preparing his resignation from RED, sifting through old diary entries of his, and trying to drink the pain away.