Here are a few concepts I considered.
NG codes as a motive.
Everyone would be given a Forbidden Action and would die if preformed. If 2 people died, the motive will end.
Here were the codes.
OJ: Yell
Flower: Hold anything in your hands
Salt: Walk (has to jog, run or jump all the time. She can hop or stand or be carried too.)
Paper: Don't hear an insult every 3 hours. (Sleeping excluded)
Knife: know anyone's NG code.
Coiny: Touch anything warm or hot.
SB: Get Angry
GB: Give an order
TB: Disobey an order
Pin: Saying no
Suitcase: Let Nickel die
Nickel: Let Suitcase die
Baseball: Be with Nickel for more than 5 minutes
TD: Use electronics
Mic: Hear a loud sound
Taco: Let anyone see your NG Code
So what happen is Flower and Mic would be killed by Taco who became paranoid of being found out as people would suspect her for not revealing her talent. She would make a loud sound by clanging a metal pipe to the wall killing Mic but gets found out by Flower who would be killed by the pipe afterwards. Taco is then found out due to lack of alibi and rather suspicious behavior that included her being very secretive.
This would take place in Chapter 3 so they would be dead already but here's these 4.
Soap: Get your hands dirty
Pen: Take your hat off
Firey: Be away from Coiny for more than a hour
Leafy: Let Firey or OJ get hurt