Here's How it Happened!
Act 1
This started off at least a day before the murder actually took place, the culprit started working on a system to kill who he presumably thought was the Mastermind. A.K.A. Hammer. Wanting who he thought was the Mastermind to suffer, he collected various weapons such as a knife, like Gold's murder, a globe like in Moonlight's murder, a rope for Lock's murder and a hammer for the DigiBot's murder. Using the Monomono machine in the shop he also acquired a hook, pulleys, a spanner and wallpaper to complete his contraption.
Act 2
After setting up the scene, he wrote an anonymous note to Hammer, calling him to the cafeteria kitchen. Upon entering the cafeteria, Hammer saw the culprit enter the kitchen and followed him inside. When he entered the kitchen though, the culprit was hiding from him and attacked him from behind.
Act 3
The culprit dragged Hammer's body out and started to set it up with the rope he had got earlier so he could stop Hammer from moving away from the zone that he had set up the knife, hammer and the globe and stayed hidden, awaiting someone to come through the door so he could release the spanner that was holding onto the railings behind the door and finish the job.
Act 4
After a few minutes, Hammer regained consciousness and must have been dazed by the attack. Finding out that he was restrained he must have started calling for help, not knowing what would happen. At this point, Samurai and B must have been nearby and opened to door to see who was calling out for someone and why. That was the culprit's trap though.
Act 5
Upon the door opening, the culprit released the trigger and sent the hammer, globe and knife at Hammer, leaving him with no chance of survival from the three attacks. This left everyone else to believe that Samurai or B was the true culprit behind Hammer Head's murder.
I must admit you almost had us all fooled...but we saw through your tricks in the end...Nimbus Darkness!
"Have any counterarguments, Nimbus...?"
"Feh...don't act so confident...you have yet to prove this is the true scenario..."
Figures that he wouldn't give up so easily....
Nimbus- Foolishness!
Nimbus- You have no proof!
Nimbus- You can't execute me!
Nimbus- You dare challenge the great ninja?!
Nimbus- Impossible
Nimbus- I laugh at your accusation
Nimbus- You shall burn in a thousand hell-fires!Enough!
Nimbus- Come now...what proof could you possibly have that proves this is the correct scenario?
Mysterious Noises!
"Remember whenever someone entered or left the cafeteria, we'd hear a clink noise? Now apply a bit of logic to that: it shows that opening the door isn't enough to trigger it, it had to be triggered by hand or it wouldn't work! Meaning...the culprit couldn't be Samurai or B!"
"Wah....?!" Nimbus gave a stunned expression.
"Alllllrighty then!" Monokuma put a paw up. "Seems we've reached our conclusion! In front of you are the sixteen ballot buttons, press the button of who you believe the culprit to be, majority wins remember! So will it be right? Or will it be oh so wrong?! It's ballot time!"
As the final button was pushed for voting, the huge slot machine appeared with 'Voting Machine' in red on the top. The three dials began to spin quickly for another few seconds before all three stop on Nimbus' face causing a red 'GUILTY' sign to flash and the two bouquets of flowers to pop out the sides.
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"Here we go, number five and it's another good guess. The one that triggered the machine that caused Hammer's demise was none other than Nimbus!"
"N..no..." Orion whimpered, her normal voice returning. "How could this be...? I thought you..." Her speech pattern was going back to normal also.
"...I'm sorry, Orion. But I had to take my chance of getting us out of here, framing Samurai and B? That was just a failsafe..."
"Why did you think Hammer was the Mastermind?" B asked him.
"Why? Something about Hammer just always struck me as...off..."
"Off? What do you mean?" Olive raised an eyebrow.
"I checked the secret archive that Beau told us about and studied up on what she had learned...the mastermind...for some reason reminded me of Hammer, but I never figured out why..."
"So, I was the reason you did this...?" Beau asked.
"No. Don't say that, never believe that you are responsible, I did this for everyone, I thought that if I killed the Mastermind we could all escape, together...but, obviously, Hammer was the wrong one..."
"Nimbus...you were trying to protect us..." Serenade said quietly.
"Yes...ironic thought, isn't it?" Nimbus smirked. "I was the one who thought you were the culprit in the first ever class trial and now...you're the one that managed to catch me out in the class trial" He gave a small laugh.
"H-how can you laugh at a time like this?! You're gonna be executed!" Samurai reminded him.
"True, but at least I died trying to help everyone, right? I only didn't say anything because I gave into the fear of death...I apologise..." He gave a bow.
"Nimbus..." Serenade's ears drooped.
"You were a Nimrod...but at least you were a good soldier..." Grimdark gave a salute.
"Out of us all...Nimbus was one of the most likely to get out...and now..." B couldn't finish her sentence.
"Hey, Monokuma, can we just get this over with? I don't want to hold any of these guys up any longer...." Nimbus sighed.
"No..." Orion said. "Please do not go, Nimbus..."
"Orion...I'm sorry...but I must go now..."
"Alright then! I guess that means it's time for the main event!" Monokuma stands up.
"You do not understand Nimbus, you never did understand my words!"
"What are you talking about?" Nimbus raised an eyebrow.
Is Orion gonna....really...NOW OF ALL TIMES?! Is she insane?!
"I've come up with a very special punishment for the 'Super High School Level Ninja' Nimbus Darkness! And this is a really good one as well!"
"Nimbus, why did you never realize it?!"
"Let's get the ball rolling!" Monokuma started to spin.
"I love you, Nimbus!"
"W-what, you're telling me this now?!" Nimbus stood aback.
"You seriously didn't figure out the most cliché final words...ever?" Grimdark said.
"Orion...I lo-"
"It's PUNISHMENT TIME!!!"
"No, wait-!"