There's a big difference between Kate and Buffy. Buffy doesn't kill those that aren't doing any harm. Hell, sometimes she doesn't kill those that do harm, not if she thinks there's a chance for redemption. However, Derek can't possibly know this given what they've said to each other and Buffy doesn't know that she needs to make any distinction.

A slayer. She'd insist there's a difference. Riley and Sam are hunters. She's got a bigger responsibility all around. Not that she's belittling what Riley and Sam do. They're amazing and they don't have to do what they do. That makes them amazing. Buffy, in so many ways, never had a choice.

She listens intently as he describes his glitches to her, describes the person that he is in them. Her brow furrows a little. "Okay, don't freak out on me, but are you drinking blood in any of the glitches? I'm not going to go all terminator on you. Swear. My last two boyfriends have been vampires."

Yup. This is what you get when you're vague.