The more I think about it. The more it becomes rational that Buffy should have killed Faith. I do not see a problem of one slayer killing another slayer. In fact, Faith was not that important, and it would have made Buffy a more interesting character if she just killed Faith. Anyway, they could have time to develop a new slayer for Buffy to train to be a good slayer. Faith, really, she is just a dime a dozen.