Tragedy Looper - GAME 2 LOOP 1 DAY 3 - Thwart the Mastermind

The main plots cannot trigger a loss condition except at Loop End, so they're not what caused this.
The subplots cannot directly trigger a loss condition, except if An Unsettling Rumor spiked a location's Intrigue to a critical limit, but we didn't see the Mastermind do that so they're not the cause.

No Incidents occurred today so the death is cuz of some kind of Role interaction.

The only Role that mentions an immediate Loop End is the death of the Key Person. Girl Student is the Key Person. If they die, the loop ends immediately.*

So how did the Key Person die? Roles that can kill are the Killer and the Serial Killer.

• The Killer can kill the Key Person at Day End if they're both: a) sharing a location and b) the Key Person has 2+ Intrigue. No intrigue on Girl/KP, Shrine Maiden is not the Killer
  
• The Serial Killer must kill at Day End if there is exactly one other person in their location. Shrine Maiden is the Serial Killer.

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*The Key Person role also means we're not dealing with the Light of the Avenger Main Plot, which doesn't provide additional Confirmed info about other roles. Yet.
**The confirmed Serial Killer role means our subplot is Shadow of the Ripper. There is 1 Conspiracy Theorist in the game. There are no Curmudgeons or Friends in the game.