Billy's so angry, she pushes herself up and yells back, "I wanted to be the one to tell you! So yeah, if this is how you're going to act, it's better you were left in the dark!"

The fact he is using words like we, and sister just fuel her more to be protective. Billy might not realize sometimes he has to be protected from his own actions. Getting angry at the others from Hawkins will just further harm those bonds - she doesn't want that. Doesn't want him isolated. Wants him to at least get along with her friends. She should have thought of that before.

"I don't think all of them know from the times they're from. But you don't make telling you things easy when you get pissed about everything!"