The second murder. Especially the Hope Spot, given that Pekoyama's still alive, but sure doesn't look it.
Then there's the second murderer's confession:
In the original version, Motive Rant is a bit of a misnomer, as Sonia-san rattles off their reasoning with icy calm. Shades of Komaeda's reveal in canon, but since he's the first victim in this 'verse, the others were never exposed to that. They also show No Sympathy whatsoever to Kuzuryuu, a jarring contrast with how compassionate she's always been. This Sonia has fully donned the queenly mask, and it's terrifying.
The Revision makes it worse, as it's still Sonia-san, and she hasn't fully assumed a queenly air to cover for what she's done...
The second execution: "Love Like Salt". Drawing off of Fairy Tales like Cap o' Rushes, Sonia is cast as a Queen who asks her daughters how much they love her. When Princess Monobear claims to love her more than salt, she attempts to banish her, only for her 'daughter' to unleash the Big Bad Wolf. As it strains against its chains, Sonia manages to keep a neutral expression... until she looks to her audience for some sign that they understand her motives. She doesn't find any. When the wolf breaks free, she dies screaming.
After the first Revision was "toned down" to Souda dying (as mostly himself) with just the huge "stab" wound.
In both of the above scenarios they are described as having a look of horror frozen on their face post-mortem and the room was covered in blood (though the amount of blood in the second was less than the first). Bonus points since the weapon used to make the "stab" wound (and the decapitation itself) was a revved up chainsaw (the same one which was seen in the background in one area of the game) so they weren't "stabbed" as much as they were disemboweledalive.
Now they have the dubious honor of being the first person to survive an actual murder attempt by one of their "friends", though not by much. This is not helped by Despair Fever's new side effect: They have no memory of what happened after the night of the firework show, so they have no idea who tried to kill them or why and have no clue what they have been doing for the past three days.
So who was behind poor Tsumiki's murder in Chapter Three? Tsumiki herself. She committed suicide — after infecting the Imposter with the Despair Disease.
The aftermath of the third trial, and its execution, as the Imposter hijacks Monobear's body and the game, killing Monomi as she screams for help. Just in case that wasn't bad enough, it's made clear that they know the true nature of Jabberwock, as they end the mutual killing game and replace it with a hostage situation, declaring that sacrifices are required. Nidai tries to stop him and dies for it; Gundam stays behind and is seemingly blown up for his troubles.
To elaborate on the reveal: The Impostor's mindset was changed by catching Despair fever and pulled a Face-Heel Turn and seemingly supplanting Monokuma as the Big Bad. Then they cement their position by proving they have all their despair memories and ends up killing Nidai for breaking the "no attacking the teacher" rule when the "punishment" for it is entirely up to the teacher themselves. The Impostorchose to kill Nidai knowing they didn't need to go that far just to bring on more despair.