The well-loved Chitanda's death in Week 1. Especially when the investigation discovers the letters to home and paper cranes in her room.
Jamie's breakdown when he finds her body first, which was only shown after the round ended.
When Kurumi is lashing out and blaming Ricky as a convenient target, Ricky can't even step up to defend herself, and she starts to believe that it was all her fault.
The Griffin's desperate (and ultimately futile) attempt to save his "brother" Toby from his execution and getting killed in the process.
Seth's coping methods when dealing with the deaths of the Griffin and Toby are kind of nightmarish, but at the same time it's tragic how this is the only way he can express those emotions. Then it turns out that he just can't feel, but until you realize that, it's still sad.
What about case 3 isn't?
The murder was actually assisted suicide.
Margulis and McBurn very much seemed to care about each other, yet McBurn killed him.
Then it was found out that McBurn agreed to this crazy stunt because he was put in a position where he felt escape or death were the only choices he had left.
So much confusion in this case that led to so much distrust being sown. That's going to take the living some time to deal with.
McBurn in the end was undone because he actually started feeling and outed himself because he started losing control of himself.
The case ending with a second suicide when McBurn slit his own throat.
Not to mention that, along with all the feelings going around, how the culprit was caught is almost a Karmic Death; he was cornered by an extremely devout religious person (Thomasin), an elemental magic user who was adept with fire (Lightning), and a human with nanotech implants (Yurika), all things the victim was known for. Add to this the fact that the victim died of the wound to the neck and the culprit inflicted the same wound upon himself.
How case 4 went down. The fact that Shadow attacked Rukia and her actions were in self-defense. However, he did die and the rules state that whoever is responsible is the culprit and if they do not evade detection they are executed.
Bolton hinting at his backstory mid-way through the round.
"I was about (Thomasin's) age the first time that I heard someone say people like me were going to burn in Hell. It...scared me. But it didn't change me."
Jane trying (and ultimately failing) to use the Lifey Thing on Ricky.
The fifth trial, with multiple layers of participants steeling themselves to kill their loved ones to spare them from execution and try and help people. Especially the murder log, with Ricky's realization of utter betrayal and Kurumi's attempt to explain just breaking down into apologies while she's choking the life out of her girlfriend to spare her friends, Lightning walking in on them and failing to save Ricky, then Lightning's decision to assist Kurumi's suicide and be executed in her place.
Xander and Lightning's kiss after she's confessed and before the vote starts.
Everyone knew that Case 6 was going to suck, being the penultimate case before endgame, but no one was quite ready for how much.
Togami being so shocked to discover the body of Bolton, who had made him and Jane sit down and eat dinner just a few hours before that he just slumps against the wall and passes out.
Jane's reaction to the body discovery and determination to investigate the scene.
Everything about the investigation of Bolton's room. Making it even worse is that the one who did most of the investigating was Xander who was the one who murdered Bolton. The revelation at the trial that Xander was a mole working for the mysterious "EP" makes his reaction to Bolton's Deal with the Devil all the more painful.
Clarith revealing her backstory to the others and wondering why they're all so nice to her when the people at home treat her like garbage.
Everything about the first case. The culprit didn't even want to kill the victim and didn't know she had, or that the creatures that had been attacking were actually made of her own issues and targeting the things and people she loved.
Clarith at first trying to murder Mai with her own two hands only to promise Mai she'd live and even refusing to look away from Mai's execution out of respect because she understood Mai.
Everyone trying futilely to save Mai since she wasn't even aware of her guilt and was guilty only on a technicality.
Mikaela having died collecting a flower that likely reminded her of Clarith's hair.
Church demanding the bodies be buried and refusing to let up on that.
Yuuri and Junpei hugging and letting out their feelings in the garden after the first trial.
Knowing what happened in Round 1, Yuuri's response to Clarith's "killing for love" theory in the second trial is even more painful than it would be without that context.
It's not the same, not at all, but... she suddenly finds herself thinking of Kurumi. Kurumi on the rooftop covered in blood again. Kurumi looking their former classmates in the eye every single time she brings the blade of the shovel down on them. All to keep the people she loved safe and alive for another day.
The reactions to Junpei turning up "dead" in week four, though he turns out to be Not Quite Dead.
The third culprit recognizes what he's done and wants to confess before anything happens, but he holds back and tries to cover up the murder anyway — because he also knows, and he's right, that P.A.L. "hates being bored" and finds the trials entertaining.
"Do you think, if someone came to the others and confessed, that P.A.L. might just kill another person because it got 'bored?'"
Choromatsu calling Takumi by the same kind of insulting nicknames his brothers get, saying Takumi was like another brother, and plaintively asking Cece if he can see him in the morgue after the third trial.
The fact that most of the murders were sympathetic and usually unintentional, the Champions try to prevent them to the point that rules had to be put down to keep that from becoming a Game-Breaker, and the cast has endgame-level solidarity and hope halfway through the game — and they keep on dying anyway.
Related, in any other game, Lee, who killed intentionally out of paranoia, would be treated as a garden variety culprit, and here, she's a Hate Sink for going against group unity. Given that she's a minor villain and a pawn back home with much less agency than she thinks she has, it's got to be hard on her.
Ardyn: No matter what I may become—or already have—swear to me that you'll remember I was once this Ardyn before you now. That I wanted the best for every last one of you, and would sooner die than see a single one of you hurt.
The deaths of the Sacrificial Lamb NPCs are surprisingly painful, even if the viewers only knew them a week.
The viewing log during the pirates' invasion. Jane has a Heroic BSoD recognizing Blaze Dudely as her own world's version, and when Church starts jesting about it and Ardyn calls him out on it, he snaps and stalks off with another reminder that Church doesn't care one bit about the Round 1 or 3 cast and doesn't want to.
Case 3. The culprit killed five people to bring back his kids, who he just remembered were dead that week.
In the finale, Noctis has to Mercy Kill Blaze Dudely, who's just revealed that he was forced to work for IG 7 in a desperate attempt to save what was left of his people. He stabs him through the heart just as Blaze did to Shirou, the round's first PC death.
The entire round is one - every person watching in the survivor pool knows that only two people survive, and there are a lot of castmates to the survivors in this round. They're watching how the people they love suffered, after realising they never even knew they were gone long before their seasons even aired.
The responses to the first real motive of the round are pretty upsetting, considering it's a hostage motive...and a lot of the hostages are in later seasons to begin with.
Both of the case 1 murders were entirely accidental - with one culprit desperately trying to save their victim and the other having no recollection of the event, let alone the fact they weren't even human when they did it. While the double case itself was already upsetting enough, the reveal of how the culprits are punished with two victims of circumstance was just as bad.
Luna regains her Fairytale Wedding Dress. She misses Noct and doesn't know if she'll ever see him again, so she goes up to the diner in the gown and starts dancing all by herself to the jukebox. The most painful part? Noctis is watching the reruns at home, and after everything he's already seen, this is the point where he can't take it anymore.
Case 3. Just... Case 3. Those poor Matsuno brothers...
Osomatsu:L-Listen, Karamatsu... you don't deserve to die for what you did. You were just trying to save everyone and Wan-chan, and I've... a-all I've done is make everyone mad and make a mess of everything. I.. I-I'm just a waste of space, but I don't want to be anymore. So.. switch places with me. Let me prove t-to you, and everyone else that I can be useful for once. [...] I-I... I can't.. I-I don't want to live if I-I have to watch one of my p-precious little brothers die. P-please just... just let me do this for you. Please....
Not to mention Choromatsu watching this all unfold back at home and completely breaking down.
Zombina's letters she wrote shortly before her death are sad enough, but then her letter to Karamatsu has her confessing that she's in love with him. We only find this out after he already helped her kill herself. And thanks to certain events, he never even gets to read it...
And the worst part? It's All for Nothing. Karamatsu ends up dying horribly the very next week.Ouch.
The aftermath of Case 4 has just about everybody having some kind of breakdown. Especially poor Akira and Kaiba.
Jane admitting that something is keeping her from hating Xander like she "ought to," even if her memory of him is blocked.
The motive for week 4 is hard enough to consider: the chance to change a single past event, preventing it entirely. Then Pyrrha ends up getting a Penny doll out of the gacha...
The entire Week of Terror. The characters awake to memories of emotionally draining cases that left them unsure of who to trust, Jane discovers evidence that Xander was the Champion Mystery, the motive has their captors kill one MIA Champion every day on a live stream, Xander's mental conditioning rejecting the claims that he's their amnesiac friend starts to take a harsh toll on his mind and body, and the final hostage? Jamie and C.E.C.E., with their Collective Identity as Lady Consequence revealed just as they're killed and Xander's amnesia finally breaks. Whatever E.P. did to Xander gives him a Psychic Nosebleed and eventually makes him faint as he fights back. And that's all before the murder.
Church is so distraught over Ardyn's death that he, in his grief-stricken fury, tells Choromatsu to kill himself the same way he killed Ardyn. Choromatsu complies, because he was contemplating killing himself ever since he murdered Ardyn, who had done his best to serve as an emotional rock for the other Champions, and even thinks that he should have done it sooner, only held back by his fear of Dying Alone. His last moments are spent pleading assurance from the survivors that they'll make it through to the end before the last of the medicine works its way through his system.
Church's speech to C.E.C.E. about identity and the choice his team was hit with in the endgame investigation.
"I'm not the Alpha anymore, and I haven't been for a long time. I've been told by several people that there's no real point in my wondering who the Alpha was, what he was like. Because I'm not him. Because I'm Leonard Church, and people love me just the way I am."