- Sakuya sacrificed her blood to her master, dying in the process?
- Sakuya tried to fulfill her original mission as a vampire hunter, forcing Remilia to strike her down?
- Remilia wanted a snack and missed, severing a major artery.
Actually my speculation is that Sakuya was perhaps severely injured somehow and was pretty much going to die. In an attempt to save Sakuya, Remilia tried to turn her by biting her. Seeing how it isn't working, well you can see...