[This may sound crazy, but I actually kinda understand why Rose shattered Pink Diamond. To me, it seems that while most Gems could be shown the values of Earth, the Diamonds are the only ones that can't. That they're just too stubborn to think that Earth would be worth anything other than being used for the Cluster. Rose may have tried to show Pink Diamond the value of understanding Earth and not destroying it, but I have a feeling Pink Diamond didn't care, and the only choice Rose had was to shatter Pink Diamond. The reason she would hide it from Steven (As shown in Bubbled, that the others knew about it), was to preserve his innocence, hoping the others would be able to reveal the truth to Steven when he would be ready for it. In fact, it kinda reminds me of the end of Wander over Yonder
Yeah, Wander didn't kill the villain, but WoY barely dealt in anything dark, and never had something akin to a huge war. Considering Rose probably had no other option, she had to do the one thing she never wanted to do.]