Complex sentence: Incredibly, Buffy still doesn't believe that Warren deserved to die for Tara or Katrina's murder, but she and Xander both agree that Jonathan and Andrew don't deserve to die since they never killed anybody.
Split into simpler sentences: Incredibly, Buffy still doesn't believe that Warren deserved to die for killing Tara or Katrina. Yet, Buffy and Xander both agree that Jonathan and Andrew don't deserve to die since they never killed anybody.