Got some new theories. Could be proven wrong. We'll see.
WARNING! MAJOR SPOILERS!!!!!!
1. White Diamond killed Pink.....for some reason. Some bullshit power struggle thing, and she's jerking Yellow and Blue around.
2. Yellow killed Pink....however I do believe her grief over Pink's death is genuine, so my guess is that she got into an argument and killed her by accident.
Either way, since Rose was already technically a war criminal by that point, it was easy enough to pin the blame on her.
But there's one thing that bugs me....if Rose was innocent, why would she let her own comrades think she killed Pink? What could she possibly have to gain from that? Or was she locked out of the loop and didn't know everyone thought she did it?
Let me know what you guys think.