User blog comment:Mellowix/Theory: Possable motive if Present Mic was the traitor./@comment-44861925-20200418191749

This seems like a good argument, but I would like to add something.

The heroes are doing the opposite of liesurely watching their students get hurt. They are worried, 24/7, about the well being of General Studies, Business Studies, and so forth. This means that they care about their student mass to the extent that they would have someone almost die for the students, such as Shota Aizawa or Toshinori Yagi. But I agree that Oboro Shirakumo died way too young, but this was just fate and had to happen to make the storyline make more sense.