Kaguya getting killed by Brolli through an air embolism in episode 10, and no her being immortal does not make okay. Her pained face when she is gasping for air is particularly heart wrenching.
The backstory of Vic Chaos and the truth about Arturo and Yuuka. If you didn't hate Arturo the first time, you REALLY will, this time.
What's worse is that Yuuka's worst fears came true. That not only did she get blamed for this incident but also had her flower field, which she let Arturo rape her to protect, burned to the ground. And if that wasn't bad enough she was trying to amend her mistake by letting the human village children survivors live there. Even Vic was horrified when he saw the death he and Mokou caused.
And also the Fridge Horror of what he became. He cannot even sleep, fearing Yukari may visit him again. There is absolutely nothing he can do, because who would believe that he went to a fantasy realm and became a killer? People would treat him like a lunatic hobo because the truth would be so outlandish that it would be impossible to find someone that would actually believe in him, not even the tin-foiled conspiracy theorists. Even if Word of God says that he would work to convince kids to not accept Yukari's offer, what exactly he can do about it? It's pretty established those kids who become gappies are nearly hopeless and why should they listen to a lunatic hobo?
A mild one but in episode 12 when Tenshi reveals to Momoi that she hid that she knew that Brolli was a Gappy from him, in order to manipulate him into involving all of Gensokyo in an incident. She starts crying, and admits that she feel remorse doing this all to Brolli, who she considers a friend.
The destruction of the Scarlet Devil Mansion and the outcome of the battle between the Diamondback beasts and Flandre. To elaborate, Hydrus orders the mansion destroyed so they can pin Flandre under the rubble, and when Flandre screams this voids spell card rules, Lepus hypnotizes her and screams, "Screw Spell Card Rules!" While she's stuck, the rest of the beasts gut the entire mansion from the inside, crushing Flandre. The rest of the Scarlet Devil Mansion crew, especially Koakuma, are devastated.
Brolli finally calling home to his parents, only to realize going home is no longer an option without serious consequences: his Cat died since he left him unattended, he has been expelled from school and wanted for truancy, and his parents think Brolli just ran away to Japan "for fun," effectively disowning him.
Fridge Horror: It is implied that his parents didn't even care about his own son.
Nitori giving into Brolli's demands and giving away the weapons of the Kappa Valley so he can make more swords, even though he has more than enough; as Brolli walks away, even Nitori thinks they're doomed.
For some, seeing Youkai Mountain turned into a war zone.
When it turns out that killing Brolli would resolve the incident, virtually every character is willing to kill himwithout question, even Reisen, who must kill him to save herself and their child (knowing he'll go to Hell) or die with him. Then Reimu of all people tells them to stop.
In the wide scope of things, the Tenshi vs. Brolli battle is sad because Tenshi used to be one of Brolli's best friends. Imagine if the person you used to go fishing and playing with caused an entire war to get you killed, and that sums up the battle in a nutshell.
The fate of the Kappa Valley, with a dash of horror. To make matters worse, the dead kappa at Nitori's feet is the same one who told her not to give her stuff to Brolli. To make things infinitely worse, the team that comes to save the survivors are forced to kill the survivors when they start attacking Brolli.
Sanae's betrayal, which often causes most viewers to hate Sanae afterwards.
The Myouren Temple ambush. That's right: the Myouren Temple, led by Byakuren, who is known in fandomnote canonically, her friendship level is only Medium as one of the nicest characters in Touhou, is forced to gun down Reimu, Sakuya, and Youmu before they save Brolli, who poses an existential threat to the youkai. And in a bit of Fridge Tearjerker, it seems Byakuren staged the hostage situation so Reimu and co. would not find out the truth, so shooting them was only a last resort.
The final Hope Spots before it's absolutely obvious Brolli is going to die: Momiji and Aya make it in time to save Brolli, but Momiji's eagerness to resolve the incident has her shooting Tenshi before realizing it will activate her last spell card, meaning they cannot save Brolli. Then, Brolli calls for for Yukari, but she refuses to save him for causing the incident, then taunts him by revealing Jack Diehard is his replacement, who is too stupid to save him. She departs by saying, "Say hi to your cat for me." After that, survival is no longer part of Brolli's to-do list.
Brolli's actual death. Whether or not you think he deserves it, the reactions from Aya, Momiji, and Reisen do not help one bit.
Nitori returning from Youkai Mountain to frame Aya and Momiji for friendly fire when they were really just trying to save Brolli.
Brolli's trip across the Sanzu River, where he realizes he left behind Reisen, he will go to hell, he loses his beret after Komachi said he could keep it, and when Komachi comes to collect the tax based on how many lives he's touched, he can only give her a thirty mon coin (in comparison, the worst criminals get at least 10000).
Watching Brolli's beret sink into the Sanzu River gets many viewers, and the music really drives home the fact Brolli's end is near. Though, some of them joke about how people feel more sorry for the beret than Brolli himself.
Reimu, who absolutely hates Gappies, actually mourns Brolli for his attempt to do good. Meanwhile, Marisa, who welcomed Brolli into Gensokyo shrugs Brolli's death off as another expendable Gappy Stu.
Aya and Momiji are dishonorably discharged from the tengu army and Aya loses the Bun Bun Maru Press after being framed by the kappa for treason.
Nitori is treated as a big hero, and the only survivor to know that Nitori is responsible for the Kappa Valley massacre is called a liar.
Sanae realizes she is indirectly responsible for the deaths of thousands of tengu and kappa in order to save Gensokyo and her shrine from Brolli, only to learn from Tenshi he was an ordinary kid like her.Even worse, Sanae can't do a thing to Tenshi or else Tenshi will reveal the truth about the incident to everyone. If the truth got out that thousands died to kill an innocent boy, it would cause an uproar that would make the previous incident look tame. In other words, Tenshi will get away with both causing the incident and indirectly dragging Sanae across the Moral Event Horizon with her, and there's nothing she can do about it. To twist the knife, it turns out Kanako and Suwako were the ones to convince her to do it in the first place.
Tenshi, despite her smug satisfaction for outsmarting Sanae and the Myouren Temple, didn't want to kill Brolli. Brolli was one of her friends, but she saw his death coming anyway for being an untrustworthy Gappy Stu. So, she killed him to give him a dignified and heroic death before the rest of Gensokyo could plot a much more sinister death.
The Myouren Temple successfully convinces Remilia that the celestials really were holding Sanae hostage and that Sakuya's belief that the Moriya Shrine and the Myouren Temple were co-opping the incident is a delusion. In a Freeze-Frame Bonus, looking at Byakuren's face reveals she ultimately regrets her decision, and because Tenshi has blackmail on the Myouren temple, there is nothing she can do.
Reisen crosses the Despair Event Horizon after Brolli's death, confining herself to her bed and, despite mentioning how much she loved being in Gensokyo rather than the Lunar Capital, wishes she never left. Then, just when you think nothing worse can happen, it gets infinitely worse: because Brolli is so much more powerful than Reisen, Eirin informs Tewi that Brolli's Marty Stu DNA is trying to override hers, eventually causing her to miscarry their child. Since Tewi had spent the movie teasing Reisen over her romance with Brolli, Eirin forces her against her will to deliver the news. The last shot we see is an outside view of the House of Eternity seen for the page image (originally used in a gag when Brolli first slept with Reisen and presumably got her pregnant), leading to this Fridge Horror: we don't know if Tewi told her the news or if Reisen found out on her own...
When it finally cuts back to Brolli, he is so far past the Despair Event Horizon that he is begging to be sent to Hell. Shiki then reveals because he actually feels regret and remorse over his actions, he gets one more chance, just as long as he realizes that it's not what he has, but who he is that matters. Unfortunately, just as he's being reincarnated, Brolli can't answer. After the atrocities he realized he just caused, all he's left to do is wonder, "Who am I?," a little too late.
For many viewers, the Downer Ending was already bad enough, but some viewers said Diamond In The Rough crossed the line after adding the Inferred Holocaust factor: it's bad enough Brolli died and led to the downfall of so many people, but Gensokyo is in ruins, its factions have been greatly torn apart, and this cycle of destruction is not only going to continue, but it's going to get worse...
When looked at from a wide view, the fact everyone is responsible for Brolli's death and the incident that caused it. Initially, Yukari promises Brolli power, which lures him to Gensokyo. Once he's there, Marisa entices Brolli into selfishness and Reimu pushes him away from doing the right thing. Then, Sanae lures him away from Reimu by being a fellow outsider, despite her lack of knowledge, and Reisen lures him away from Yukari, teaching him that maybe giving into the luxury of Gensokyo is a good idea. As a result of Brolli's resulting hedonism, Aya is forced to publish a news article on Brolli to detail his weaknesses to protect the Tengu Village, the Moriya Shrine goddesses must employ Tenshi and the Myouren Temple to protect their shrine, Tenshi must make a giant incident to get back at the people who want to kill Brolli, the Myouren Temple must mask the fact they're trying to kill Brolli to keep their followers, and Nitori is too much of a pushover to say no to Brolli taking the Kappa Valley's weapons. Ignoring that Yukari was the original cause by gapping in Brolli, the ensuing chaos of the ending is everyone's fault.