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Tear Jerker: Super Dangan Ronpa 2
The climax of the first trial, made worse because the culprit had one of the least selfish motivation of any killer in the series: not only did Teruteru attempt to stop Nagito and end up killing Togami by mistake, the reason he wanted to leave the island was to return to his ill mother. Especially upsetting because most had regarded the culprit as a one-note Joke Character until that moment, the revelation was enough to move several characters in-game to tears.
I'm sorry... everyone...
I didn't want to sacrifice anyone. I just... had to do something...
And yet... I ended up killing Togami instead of Komaeda...
Oh. I see.
It's me... I'm the one who went crazy.
On a related note, Nanami points out that it's most likely that the reason Togami died instead of Nagito was that, thanks to his night vision goggles, he saw Teruteru moving under the floor and moved to shield Nagito from the skewer...because he promised everyone that no one would die under his supervision.
The second execution: Peko Pekoyama. Just before it begins, Kuzuryuu breaks down into tears and begins screaming that he never wanted Peko to be his tool - he just wanted her by his side. Monobear takes control of Peko's body and forces her to fight an army of robots while Kuzuryuu tries to get to her, and she ends up slashing his face. Horrified, she cradles his body...and then the robots overwhelm her. Later, the characters discuss how lucky it was for Kuzuryuu to have gotten away in one piece, and they realize that Peko protected him as much as she was able to before she died.
Before the third execution, Komaeda expresses honest anger with the murderer for killing purely out of Despair rather than to inspire Hope. Mikan responds he's got it wrong: she killed out of love. When Komaeda's shocked by this, she talks about how someone accepted her for who she was, with all her flaws... and adds that she feels sorry for Komaeda, who's never felt that kind of love.
The fourth trial and execution: Gundam Tanaka and Nidai Nekomaru. When the entire group was left with nothing to do but starve to death, these two decided to try to "kill each other" just in order to have a murder committed, and therefore get food to all the rest of the group who needed it. By the end of the trial, every character seems to understand that for Gundam and Nidai, this was actually their sacrifice to save the rest of the group from a prolonged and painful death.
Not to mention Gundam saves his hamsters by placing them out of the way before facing his own death.
His hamsters cry for him.
And then his dead pets carry him to heaven...I'm not crying, I'm not crying...
Sonia's reaction to the whole thing is saddening as well. Gundam is arguably the student she is the closest to and when she finds out he is the murderer she is completely shocked. Even so, she cries and pleads with Monobear to not execute Gundam because she understands that he did it all for their sake. Gundam's last words are also what motivate Sonia to continue surviving and to not give up on the investigation in Chapter 5.
Akane is usually the toughest and most aloof of the students, and the only time we've seen her cry before was when she was infected with the Despair disease. So seeing her genuinely sobbing while cradling Mecha!Nidai's head was especially heartbreaking. It gets worse when you learn about her life growing up. Akane never bothered to make close friends, or even learn any names, because there was always a chance they were going to die the next day. But at this point she had become extremely close to Nidai. She allowed herself to become close to another person, only to lose them. And it was soon after she almost lost him the first time too.
The fifth trial and execution: The kids determined that Komaeda committed suicide, and we figured that was it—vote him as the culprit, no one else has to die. Except that he actually set up an awful plot which created an accidental culprit, so even though the entire plan was set up and carried out by Komaeda, the actual cause of death is laid upon none other than sweet little Nanami. This was all part of Komaeda's plot to kill the rest of them so that only the traitor would live, but Nanami wouldn't allow that—as soon as she figured out the truth, she decided to sacrifice herself to save her friends, and that was truly one of the most unfair and heartbreaking moments of the entire game.
When you find out that Komaeda didn't just have a moment of trollish insanity when his aim was to kill everyone but the mole: He learnt that everybody, including himself, were the remaining members of SHSL despair and that the mole was from a foundation trying to purify the world from despair. Out of the horror of this truth, he aimed to kill himself and the rest of the people who once bought despair into the world, leaving the one person (the mole, who did not cause any despair) alive. Komaeda's mental condition made him take this news so badly, that he wanted everybody who caused such despair to be gone from the world. He had good intentions, poor guy.
In Chapter Six.
One of the reasons Hinata's so wary about the Forced Shutdown is because it'll get rid of everything in the game world - and since Nanami was an AI who never exist outside of the game, they'll forget her entirely.
Also during the final trial, the main reason why Kuzuryuu is reluctant to invoke the forced shutdown isn't because he doesn't want to go back to being Super High School Level Despair (although he's also against that)- it's because he wants to see Peko again.
Everything about Izuru Kamukura, once you get past the Nightmare Fuel. He started out as an average reserve student who just wanted to gain a talent and escape his second-class status, and ended as a mass murderer, bitter and bored with existence. Chapter 0 (the short scene at the end of Chapter 5) shows just how empty and apathetic he's become, with no trace of the cynical but good-hearted Hinata left.
Pretty much every character's backstory. Most notably:
Peko Pekoyama: abandoned as a baby and raised to think of herself as only a tool to serve Kuzuryuu.
Mikan Tsumiki: bullied so mercilessly that she thinks degrading herself is the only way to gain attention. It's implied that she was in love with Junko, who's the one behind the killing.
Kazuichi Souda: bullied at school to the point that he tried to completely reinvent himself into a tough guy jerk, because that's what his bullies were like and he associates that behaviour with strength. He was also implied to be abused by his father.
Akane Owari: grew up in poverty and has worked in jobs were she was sexually harassed, all to gain money for her several siblings. In chapter 4, she says that the cheap room in the funhouse is a palace compared to her home.
Nagito Komaeda: his life has been a constant cycle of bad luck, good luck which has included him being kidnapped, then winning the lottery, then losing his parents in a plane crash, then gaining a massive inheritance. Lately, he's also been diagnosed as terminally ill. This cycle has driven him so insane that in Dangan Island it's revealed that hewanted to die on the island.