You know what? It's hard to really argue this one, because this seems to be another one of those ones like "did Angel really kill a Senior Partner?", or "What made it snow in Amends?". There seems to be evidence to take multiple sides of the question.
My personal opinion is Buffy had reached a point in the story where she'd decided a showdown with Faith was inevitable, and it would probably end in her having to kill Faith, much as she'd reached a similar conclusion with Angel in The Becoming.
There's this bit when Angel meets up with her at the vulcanoligists apartment:
Angel: I checked in with Giles. He asked me to back you up.
Buffy: He's afraid I'll run into Faith.
Angel: And you're not?
Buffy: It's gonna happen sooner or later.
So yeah, when Buffy went for the final face-off with Faith she took the knife, but she also took handcuffs.
Also there's this little interplay between Buffy and Xander:
Xander: I don't mean to play devil's advocate here, but are you sure you're up to this?
Buffy: It's time.
Xander: We're talking to the death.
Buffy: I can't play kid games anymore. This is how she wants it.
Xander: I just don't want to lose you.
Buffy: I won't get hurt. (reaches into a weapons cabinet)
Xander: That's not what I mean.
It would appear Buffy would prefer not to kill Faith, but circumstances may demand it based on her need to do what she sees as her job, protecting innocents from the supernatural menace. Xander doesn't want her to cross over into that gray area where she becomes a killer rather than a protector.
Here's the thing though; because the question is possible it's hard to make the leap, and just black and white call Buffy a murderer.