So she's not only insane, she's a hypocrite. She's been "watching" him all this time, no different than how the MC has been watching the other girl. All he's been doing is looking and admiring her, and it doesn't seem like he's ever had any intentions of doing anything but looking. "People watching" is an actual activity many people take part in, and admiring someone from afar is not much different. Calling him scum when she's doing the exact same thing is despicable.