I think Bea will need to die at the hands of the kid in Kill Bill Vol 3. But I think it would have to play out kind of like Highlander might where a third character is introduced as the real antagonist. Bea would take the hit fore the kid, and the kid would perform Bea's coup de grâce, but only to defeat the antagonist. The kid wins, Bea does with honor, and the antagonist dies.