The Light Novels are known for the dark themes and political tones in an apocalyptic world where the Gastrea virus has wiped out most of humanity and turn them into Eldritch Abomination, so it's no surprise that Black Bullet will have scenes that will break readers into tears.
Kayo Senju's death for starters. She manage to fend off a swarm of Gastrea by herself. However, the virus in her body overwhelmed her beyond critical value and her body was slowly being eaten by the Gastrea virus and turning her into an Eldritch Abomination. Kayo wanted Rentaro to end her pain and suffering and Rentaro broke down in tears when he Mercy Killed her.
Kayo Senju: Rentaro-san, I think that next…….there’ll be countless trials awaiting you. If you ever lose direction, get lost in the darkness, please follow the direction of your heart………light…….follow the light……….Rentaro………..please save…………………….this world.
The eventual fate for most cursed children is that they since they are born with the Gastrea virus in their own blood, the eventual fate for most of them is that they'll become Eldritch Abomination like most victims of the Gastrea virus. The treatment and medication for the cursed children are only Hope Spot, as it only suppress and slow down the Gastrea erosion rate in their bodies, not stop it; and is only manageable under 50%. Once the erosion rate reaches above 50%, then the cursed child have no way to maintain her human form and usually Mercy Kill is the only option. To make matter worse, overuse of their superhuman abilities or pushing their abilities to beyond their limits to obtain new powers accelerate the process faster. No wonder why so many people have some justifiable hatred towards the cursed children.
Ever wonder why Enju Aihara always maintain a cheerful and kind-hearted little sister personality? This is mainly because for starters, she's mostly unaware of her eventual fate of the cursed children if the erosion rate isn't left unchecked. She is highly aware that she needs to take a shot every day, but she doesn't know why she needs to do so. Rentaro wants Enju to have a happy life while her time as human is left, but telling her directly about this fate would most likely break Enju in tears or change her personality forever. Kayo's knowledge of her own fate as a cursed child is one of the main reasons why she is willing to accept death as her fate and have a more stoic personality.
Even more tear breaking is that Rentaro sees Enju as a little sister, is emotionally attached to her, and have to pull a Mercy Kill on her eventually.
Enju's status as a cursed child was eventually found out in the entire school she attends to and her classmates and teachers alike started to bully and jeer at her is very heartbreaking.
The treatment of the cursed children in society is also very tear breaking. While some of this may be justifiable since many of those who surived the Gastrea war lost many of their relatives and the fate of the cursed children, how society treat them is sickeningly in Jim Crow law levels. Public flogging and lynching are relatively common and police are usually apathetic when a cursed child is murdered or beaten up. Cursed children live in segregated societies where normal humans live in normal lives while the cursed children are forced into poverty and live in a life of crime. Government persecution against families with cursed children are common and families are either forced to leave their child as orphans or flatout kill them. Abortions and Infanticide on cursed children are common.
The only way a cursed child can leave their hopeless lives is becoming initiators for civil securities; but even so, how the cursed child isn't treated any much different aside from having a better life. Most promotors normally treat their cursed child initiator partners as either slaves or tools (as Kayo Senju points out how Shogen Ikuma treats her and other initiators). Rentaro, however, treats Enju more like a little sister to him instead.
There is still some hope for the status of cursed children in society. Seitenshi, a Reasonable Authority Figureis pushing for civil rights for the cursed children. What makes this is even tearbreaking is that her loyal aide, Kikunojou Tendo, who fought in the Gastrea war, holds a lot of political influence in Tokyo and thus pushing for civil rights for the cursed children to be difficult.
Rentaro's "death" when he took the bullet for Enju when Kagetane was about to shoot Enju. Enju literally broke down in tears and cried for Rentaro. Luckily, he gets better.
It's even worse in the manga, where Rentaro has an out-of-body experience and witnesses a vision of what would happen if he were to fail: the Tokyo Area being destroyed and Enju refusing to leave until she finds him
At the end of the first volume, Enju's diagnosis:
Patient: Enju Aihara
Doctor: Sumire Muroto
Gastrea erosion rate 42.8%.
Percentage until complete collapse 7.2%.
Doctor evaluation: Dangerous domain. Didn’t inform her on the value to avoid her being stimulated. By regulations it’s suggested to change initiators.
The following advice is as a friend, not as a doctor.
Don’t let her fight anymore Rentaro-san.
In volume 3, Rentaro and Kisara started a school for a group of cursed children in order to create hopes for their own future. About a half volume later, the said group of cursed children are later murdered and lynched by a group civilians that Rentaro supposedly protected.
The anime is less brutal and instead have the group of cursed children bombed. What's even more tearjerking and heartbreaking is that Enju finds out that her new friends were killed and broken down in tears. Rentaro even tried to restrain her because he knows her knowing that her friends would be too much of an emotional burden for her. And Enju really wanted to show her friends the Tenchu Girls anime series that they will never see because they're killed off.