It's not your fault! It was his fault, for making such a dangerous experiment. And my fault, because I couldn't--

[He hadn't been able to kill her, so everybody else had died. Even now, he doesn't want to kill her, even though he blames himself and knows that would have been the right thing to do. But she's Shirley. She's important to him. They were supposed to always be together.]

It wasn't yours.

[It was true that she'd taken that potion, but it's hard for him to blame her.]