Why would Senpai ever run after a masked killer and rip the mask off?

Because the game is still in development, Senpai’s personality is not yet set in stone.

It is likely that some NPCs will have unique reactions to seeing a loved one get murdered. For example, if you murder a character’s sibling right in front of them, they may attack you regardless of their personality.

I imagine Senpai as the type of person who would normally be gentle and prefer solitude, but upon witnessing a masked killer, would feel enraged at the cowardly act of killing another person while hiding one’s own identity, and would unmask the killer without regard for his own safety – especially if the masked killer just murdered his childhood friend or little sister.