This all happens because of the student council.
Tenko went to ask Shuichi and Maki to tell Angie to cut it out and stop using a motive since that obviously wont turn out well. She only finds Maki in that courtyard however.
Shuichi meets up with Angie in the warehouse during nighttime about fifteen minutes after Maki and Tenko are done talking with her because he wants to see the book before they do the ritual to bring Rantaro, but he makes it seem like he wants to join the student council so that Angie will let him talk.
He finds out about the steps to the ritual and how they include burning the book as the final step, however he also finds something Angie missed.
A carbon monoxide warning.
He trys to bring this up with Angie saying how they shouldn't go through with the plan as burning the book would kill everyone in the room when they do it. Angie does not listen and instead fights him asking if Tenko put him up to this.
Shuichi grabs the closest thing to him and hits her in the stomach with it, however the closest thing to him was a small six inch sickle meant for gardening.
She screams incredibly loudly and it alerts the only other person in the main building, Korekiyo, who had been at the small sitting area trying to read a book he found in his lab.
But instead of helping Angie or pulling Shuichi away, he congratulated Shuichi and asked if he could help finish her off.
When Shuichi reacted in shock, Korekiyo explained how he wanted to send friends to his now deceased sister, and how if he could help that they could both be the blackened killing all the other girls for more friends as they escaped, Shuichi allowed to go home and forget everything and Korekiyo allowed to get out and finish his goal of a hundred friends. Shuichi agreed somewhat and said that they should take her to Tenko's lab as she mentioned her before she passed out from the blood loss.
He also took a bottle of eye drops, some salt and the sickle before he left.
When they got to Tenkos lab, Angie was already dead.
As Korekiyo set her body down, Shuichi stood behind him, and plunged the sickle into his chest.
He then moved his body to look like he stabbed himself and used the eye drops to make Korekiyo look like he cried to protect him from criticism. He used the salt to make it look like a ritual killing.
Then prayed to whatever god would listen that his immanent execution would be quick and that Angie and Korekiyo would be able to forgive him one day.
He then left to try to sleep for likely the last time in his life.
The investigation happens after Tenko, Himiko,and Maki find both bodies.
The investigation doesn't go very well since Shuichi is refusing to actually go to the crime scene.
Kaito tells him that they all rely on him and that he should just go face his fears.
Shuichi says that they should all stop relying on him and that they're going to need to do it all themselves.
Due to them not really having a detective for this, everyone is working overtime to try and pick up on everything.
The trial begins with Miu asking why Shuichi refused to help, Kaito is the one to say something however and he tells her that Shuichi just didn't want to encroach into Tenkos space.
This shuts up that avenue of discussion very quickly.
They first assumed Tenko as the killer however this quickly changed to Korekiyo as the killer, and it flip-flops between those two for most of the trial. It's only after Kokichi brings up the possibility that they could have different killers that the topic changes.
Shuichi asks if they can have two blackened, Monokuma says no and brings us the first come first served rule.
Shuichi rephrases and asks if two people can kill one person, Monokuma again says no.
The discussion continues, with Kiibo asking if everyone has a airtight alibi as the time of death was at around eleven at night. Tenko and Maki are the only ones with a real alibi as they were having a sleepover and brainstorming how to get Angie to stop using the motive.
This proves it couldn't be Tenko.
There's salt on the heel of Korekiyo's shoe which means the ritual circle was drawn after Korekiyo was dead.
This proves it couldn't be Korekiyo.
That also means we're out of ideas.
Then suspicions are finally placed on Shuichi, due to him not turning up to training that night.
The pieces are finally in place.
Why didn't he look at the bodies? It was his crime scene.
Why was he asking Monokuma strange questions? He was asking about a previous plan.
Why did he insist on everyone else being able to do it themselves? Because if they couldn't the next killer would get away with it.
All the signs point to it being Shuichi, and when asked to give his defense, to tell them why he couldn't have done it, he just laughs and says congrats. He tells them why he did it. And then he says he's so proud of them, they managed to face the ugly truth, when even he didn't want to. He says he's sorry. He doesn't say anything about Korekiyo's sister. And then he asks you to please just vote, so that you can finally get out of this terrible trial, so that you can stop wasting time on the past and move forward once and for all.
To please keep her wish in their hearts since he no longer can.
And then...
"Phuhuhuhu! You guys are great at this! You got it right! Again! Alright lets give it everything we've got! IT'S PUNISHMENT TIME!!"
And everyone just breaks. It's just like Kaede's execution in that regard.
Himiko sobs, and so does Kaito. Even Maki cries a bit. Everyone just feels lost now.
They leave the trial grounds wishing they never entered the place at all.
