(OMORI SPOILERS!!!)

You witness Mari and Sunny fighting ontop of The stairs. Sunny throws his violin down The stairs and Mari gets angry at Sunny

Mari: "WHY DID YOU DO THAT SUNNY?! THE RECITAL IS TOMORROW!"

Mari yells at Sunny as Sunny tries to get away and Mari blocks his way and Sunny ends up pushing Mari down The stairs and ends up killing her. Sunny IS panicking

Sunny: "M-Mari?! Oh no... Oh no no..."