I think you're right, Buffy wouldn't have killed her. And I agree I think Buffy would have resolved to put her down permanently somehow, as you said. Something drastic had to be done, especially if she hadn't broken down for help.She would've stopped her of course, but I don't think she would've killed her. Buffy prides herself on not being a killer. Giles killed Ben because he knew that..
She wouldn't know what to do with her though, she could try to find a magical solution to strip her powers/incapacitate her somehow. Or she might leave it to Angel to handle. Either he helps her or ..