SOOOO when the fire happened--one of the characters commented that only the fire grenades are there but there were other extinguishers earlier (cause for alarm in my head 'Why only fire grenades?'--do these things have explosives?)
When it didn't put out the fire, I thought maybe that it was going to make the fire worse and somehow one of us would have ended up killing Nagito. However, we can see that wasn't the case. So now it was a murder and I was like 'who did it' but the second Chiaki pointed out the blood stain on the girder I was like 'SUICIDE, YUP'...
FFWD to us going to his room--Let me explain the "traitor". The second I opened the box and found the diary, I knew that Chiaki was the traitor. Should have been clear to everyone I would think. So. When I rewinded my memory on how she quickly swiped her ID I figured--ah, it didn't work. Nagito doesn't know who the traitor is. Nagito obviously couldn't tell by this book either.
Then when I opened the fridge and saw the poison--I instantly knew that he filled one of the grenades with poison and that would be the cause of death.
Finally my brain settled on these facts 1) Chiaki is the traitor 2) Nagito didn't know who the traitor was 3) Nagito figured out who the traitor was by using his Ultimate Luck. 4) I hate myself for going into this trial knowing this.