Who killed Buffy

Buffy died twice. However, we have witnessed three Buffy deaths. There is also a contentious gunshot situation that is sometimes thought to be a fourth death, though I will try to explain why it is not.

One: The Master drowned Buffy during a fight in the season one finale. After he left to do evilness about town, Xander and Angel arrived on the scene. Xander quickly revived her with CPR, more or less. Two: In the season five finale, a hell-god from another dimension named Glorificus (Glory for short,) has opened a portal using the blood of Buffy's sister, Dawn. The portal will bleed an incalculable number of hell-dimensions into our own, essentially destroying it. The only way the portal can be closed is with the ending of Dawn's life inside it, though Buffy realizes they share the same blood. Buffy throws herself into the portal to save her sister's life, and the world, and consequently dies. Despite falling another 300 feet after she is dead and the portal is closed, this is noted as being a very "pretty" death, as Buffy has nary a scratch on her.
Buffy was brought back in the following season six opener by Willow, Xander, Anya and Tara, via a powerful spell that required the Urn of Osiris - a spell that had several repercussions throughout the year - and an act that ultimately set up that year's entire arc in many ways. Contentious Gunshot Death: Though some may theorize that Buffy died temporarily on the operating table at the end of Season 6 when she was shot (by Warren Meers,) her condition was always critical to stable. If she were to flat-line, the writers would have keyed us in on it. She was critical to stable in this brief period of time, before being magically healed. Though other victims of gunfire were a topic of discussion throughout the rest of the show, her bullet wound was never referenced again, downplaying the probability that she died a third time. In the musical episode "Once More With Feeling," before the gunshot incident, she had a line in one of her songs where she jests, "It's do or die - Hey, I've died twice." This one "never happened" as far as Buffy knows: She also died at the end of a season three episode entitled "The Wish," but for two major reasons that can be delved into much more than I am about to, this is never considered 'canon': 1.) We were magically put into an alternate universe where everyone - including Buffy - was very different from the characters we know on the show. So when she died, it wasn't the Buffy we know who died. She was somewhat more Faith-like in this episode, though not to a "T." She was still Buffy, but a Buffy from an alternate dimension.
2.) The episode was undone at the end, and everything that transpired is only remembered by a handful of characters, namely a vampire-version of Willow, and Anya - the demon who enacted the ruckus in the first place. So Buffy died 3 times that WE saw, but our Buffy only died twice. Drowned by The Master - S1 - Prophecy Girl
Neck Snapped by The Master - S3 - The Wish
Killed in Mystical Portal (created by Glory) - S5 - The Gift Shot by Warren Meers - S6 - Seeing Red - did not die but is debatable.