Of course, he knows there's nothing. Logically, he couldn't have stopped her-- and for all the effort Billy put in to trying to unmake the events of that day, it hadn't worked out in their favor. Far from it. He nods a little in agreement; knowing what's true doesn't keep it from hurting. A part of him will probably always wonder if he might have saved her.

"She sacrificed herself for me. I couldn't have stopped it, but... I don't know if I'll ever shake that feeling."