If it weren't for Erik asking them to, Billy would have stayed and watched, just so he didn't have to go through it again on his own. He wants to tug his hand out of Charles's grasp, tell him he's seen things no child is supposed to and this won't be any different.

But he gently shakes his wrist free and heads to the door without another look back, knowing if he sees Erik again, he'll try to stop this, do something else. And he doesn't want to be responsible for him living through it once more.