The bad feeling in her gut that Buffy had told herself couldn't possible have any logic to it? Turns out it was true. Died. As in, dead. Dean had said goodbye to her, knowing he was going to die. If only he'd known what she was, what she could do...she could have helped to save him from whatever it was.
"Okay so..." She was frowning heavily, not at all ready to acknowledge that this could be the final say in things. "How did he die? From his letter it sounded, like he knew - I mean, who was after him? Was he gambling, uh, the mob?" Buffy had no idea. But whoever they were, whoever they could send - she would have been able to beat them up. For Dean, she'd still do that. Anger started to rush through her. "Who killed him?" She wasn't even acting like it was a question that he wasn't murdered.
Buffy knew that Sam and Bobby would look at her, and see nothing much of a threat. But once she got the information she needed, she'd set off and....do something. She just wasn't sure what yet.