I need t' tell ya abou' somethin' dangerous in the 'ouse. You're the experts, you an' your brother. I'm sure you've dealt with the sort before, but once I tell ya, I gotta ask ya t' leave it alone.
[He's very quiet for a long moment, thinking back to the last few times he ran into a vampire. To be honest, it's a good thing he knows before Dean. He's way more likely to let the thing live.]
[He knows it's ironic all things considered, but he doesn't like the idea of a vampire running around unchecked. Or Rose running around with said vampire.]
[He sighs and shifts. Normally, under any other circumstance, he would have given her the third degree. But it's late. And he's tired of this house and all the shit it dumps on them.]
[He inspects, but can't help feeling like an asshole for it. Sam nods, and moves back to his original perch against the counter. They can hear the sharp dripping of the tap against the forgotten kettle in the sink. The house creaks with age. She's standing there trying to do the right thing, and Sam has no idea how he's supposed to feel.
The fact of the matter is that there are vampires who can control themselves. Who are capable of living off animals and make the conscious decision to do no harm. He's seen it with his own eyes. And that little spark of humanity inside the monster gives him hope that maybe he isn't so bad off, either.
And then he realizes just how upset he was at the idea of Rose being around this guy. And just how attached to her he is. And it scares the hell out of him.