Billy loses her in every universe. Every last one, and he's seen them all. In some, that so-called hero is the one to do it, whether by accident or intention – taking her life while trying to kill him, or pulling the trigger on her himself. The ones where he does it directly are the sweetest and the cruelest of them all, because all of those are where she's fallen for Billy, where what would be mere dreams in the other 'verses are real, and beautiful, and never-lasting.
There's universes, his favorites, where she outlives him. But never with him; always with another. No, not "another" – always with the hero.
It's irony, to be honest, how ruthless that hero is. If it's "evil" he'll destroy it without even caring to flinch. Without caring what it is or who it is. Without caring if it's her.
He cares. And there are 'verses where he dies, just because he cares. Where her eyes are cold as she draws down on him, her enemy, the villain.
Those are the verses where he truly loses her.