I know it seems a little obvious to be pointing fingers at White Diamond, but Zircon's argument for Pink's shatterer being a Diamond is sound. We know almost for certain that Rose didn't do it (unless the Laser Light Cannon projected Rose with her sword and that was what shattered her, or something), and we also know enough about both Yellow and Blue to believe that they didn't do it.

(Blue wouldn't have openly questioned the circumstances if she'd been the one to shatter Pink; Yellow wouldn't have tried to get the Rose Quartzes shattered if she didn't believe they did it)

At that point the only real suspect is White, who has the same (and probably higher) authority as the other Diamonds, and hasn't been seen in the entire show.

I think White's motive must have involved maintaining the integrity of the Gems, or of the Diamond species in particular. Pink seemed both younger (less mature) and defective (smaller than the others), so I believe she was created after Homeworld started running out of resources. Yellow and Blue became familial with her (as shown in part during Jungle Moon), but White saw her as a weakness of the Gems and a potential threat, and shattered Pink to fix the issue.

I think part of why she's absent is that she's trying to distance herself from the issue. The Diamonds never had to deal with grief and have nobody else to guide them, so they're kind of a mess. Yellow tries to act so she can drown out the pain; Blue mourns openly; and White, the guilty party, dissociates entirely, trying to justify her actions to herself.

I don't believe that any of them are truly evil, but I do think that Pink was like (ironically enough) the Rose Quartz/Steven Universe of their group, objectively imperfect but part of the family nonetheless. White made a decision based on her objective values, not seeing that the other Diamonds saw the same issue subjectively.

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