I can't believe Phos' aims have changed so much from the beginning. The writing was exquisitely simple (if a long time IRL to get there).
In order to save everyone, Phos has concluded to kill everyone. Miraculous. The face-heel turn doomsday conclusion isn't uncommon for cheaper plotlines (e.g. the overdone "machines determine the safest way to stop human wars is to kill all humans"), but getting there so that the murder of one's friends/family/everyone is actually a reasonable (if extreme) logical conclusion to END suffering for the character is extremely difficult.