Ayame's reaction to seeing her father shambling as a Kabane as the Kotetsujou leaves the station. Kurusu saying that he's "just a Kabane" doesn't help matters.
Ikoma manages to get aboard Kotetsujo with Takumi and saves the passengers from a Kabane trying to claw in through a hole while also demonstrating his invention. Celebration is cut short when Ikoma faces the passengers and everyone backs away from him upon seeing he has a heart cage just like Kabane. Even Takumi. Well, Ikoma would back away from himself too if he could as the realization his contraption only mostly worked punts him straight over Despair Event Horizon. Then he gets thrown overboard, despite Takumi's pained protests.
After saving the Kotetsujou, Ikoma has a breakdown as it pulls out of the station. He's only seventeen, and he knows that the infection is eventually going to overcome him and he's going to die a horrible, painful death and spend the rest of his days as a mindless monstrosity. He saved the Kotetsujou out of spite, but the realization that he was going to die alone finally got to him.
The breakdown is to the point he refused to take the line Takumi've thrown him from aboard the train even if it's his only chance to not die alone. Mumei tells him to stop whining and straps the line to him so Takumi can reel him in.
Ikoma's backstory. His home station was overrun by Kabane, and the adults left the children of the town behind. As a result, his sister was bitten and turned. He ran away under the pretense of calling for help, and nobody even tried to come to his aid. He eventually ran back to his sister, only to find her fully turned. He kills her with a suicide bag, tearfully apologizing the whole time. It's this series of events that led to Ikoma becoming almost irrationally angry at the cowardice almost everyone else shows to the Kabane.
At the vigil at the end of the third episode, a pregnant woman, Shino, is revealed to have been bitten by the Kabane and has to be killed by Mumei. Kajika tries to tell Mumei about her pregnancy, but by the time she's finished the sentence, Shino is impaled on a pair of swords. To make matters worse, just as she dies, Shino's Empty Eyes briefly turn human again, implying that she's aware of what is about to happen.
Episode five starts to flesh out Mumei's backstory with the introduction of Enoku, a crippled former employee of her "brother". Enoku informs Mumei that if she fails to act as her brother demands, she will be cast out like a broken tool. Worse still, his parting advice is for her to server her purpose while she is still human.
Episode 6 revealed that her life was actually a fair bit worse then we had known. When she was a little girl (well, littler) her home station was overrun by Kabane. A fearful samurai cornered her and her mother and shot her mother, who died shielding Mumei from the bullet. Only the timely arrival of Mumei's "Older Brother" saved her, even though it ultimately led to her being conscripted as a child soldier and Kabaneri. Coupled with her increasing self-doubt, it's hard not to see that her confidence was much more then a shell for a scared little girl.
The death of Horobi in Episode 8. She was Biba's loyal subordinate and happily took the black blood injection from him in order to fulfill his plans of destroying the gate and causing havoc. She did so successfully but lost control due to her restraining choker crumbling faster than expected, causing her to wantonly slaughter her comrades until she reaches Biba, who she was about to stab but stopped at the last second. Her recognition of Biba makes her regain her sanity, and then she smiles at him in relief... before he unceremoniously stabs her through the heart, putting her out of her misery. The saddest part is she knew full well she would die that day as nothing but a pawn to Biba's plans, but still went through with it.
Pretty much everything that happens to Mumei in the later episodes qualifies.
Takumi's death in episode 10, especially when you remember his initial wish was to just live through all of this.
