There has to come a point, however, when the taking a human life is the only viable solution to an existing problem. The final question, to me at least, is just what the Slayer's ultimate mission is. Was the First Slayer, and all of her successors given her power to protect those too weak to help themselves, or was she created to simply be predator whose natural prey is vampires and demons? I in no way advocate the kind of piecemeal slaughter that Blade engages in, making no effort to determine vampires from their human familiars, but there most definitely are circumstances where a Slayer's duty to protect humans combines with her mission to rid the world of demons, vampires, and supernatural enemies simply has to require the taking of human life, in exactly the same way in which a police officer, under very specific and limited circumstances, is justified in the taking of another's life in the accomplishment of his or her mission.
What the question boils down to for me is, "Is the Slayer's mission simply to kill vampires and demons, or was the Slayer line created to protect the helpless from supernatural enemies?"
If the former, then the Slayer might be required to fight blindfolded and with one foot in a bucket of sand, because there ARE evil humans, humans who have chosen to ally themselves with dark forces, humans who have been empowered in such a way that normal humans could in no way deal with them. Ignoring that fact would make their mission impossible.
If the Slayer was, as I truly believe, created primarily to protect mankind from the dark forces, things that they by themselves are incapable of standing against, then that would have to include the ability, as well as the sacred duty, to take human life under some circumstances.