Those siding with Phos don't understand: From the very beginning, she has been the conductor of her own demise. Every step she took has been one misstep after another leading to her inevitable spiral to destruction, eventually leading to this dark, suicidal Phos who wants nothing but death. She said it herself: 'Pray for me'. She wants death. It's not about the other gems anymore- and in a way, it never was.
Back when Phos was innocent, kind, and weak, she was nurturing, kind, and well-liked, even beloved. Even if she couldn't fight, she could encourage others with her kindness and her words, and was never judged by them. But she wanted to be strong, not knowing what a change like that entailed. She learned the severity of their fight, she learned the meaning of loss, and became cold and cynical and distant. This isolated her from those she once loved. Then she became secretive, and manipulative, as a result of her self-centered desire to bring back those she lost rather than focus on improving in the present, which resulted in even more loss and her diving even deeper into the pit of self-loathing.
Then, once, she succeeded. She reached the moon. She learned the truth. And she found a mean to assuage herself of guilt, even if it meant betraying Adamant, who gave them everything they ever had. Even Achmea said as much: He is not malicious, his secrets are simply part of his design. But she wasn't thinking about that. She was thinking about how she could make up for her past mistakes, and exploited the weaknesses of her fellow gems in order to accomplish this, something the weak Phos never would have done. She was all too ready to abandon more people for the sake of her own happiness.
Even Cairngorm warned her that she would keep losing people in the present for the sake of those who are already gone. And now she's here, one failure too far yet again, and the precedent of caring about others has dropped. All that remains is her desire: Her wish for an end to the suffering. None of the gems are to blame- she set herself on this trajectory from the very beginning.
