>>100196487MY THEORY
Pearl was trained by Rose in secret while she served Pink Diamond, becoming attached to her in the process. Despite her objections, Pearl shattered Pink in Rose's name with her sword, inadvertently framing her for the crime and causing Yellow, Blue and White combine their powers in a last-resort tactic to wipe out Rose and her rebellion. So now, not only does Pearl have PTSD from the war, but also the guilt of having betrayed her lover's interest and indirectly bringing about the near-undoing of her army.
ALT
She shapeshifter into Rose to commit the murder, which is why she's reluctant to do so now.
