She killed Pink diamond. It's pretty obvious now. But, why she did it is still a bit confusing. My thought is she wanted earth to use for building armies and colonies that would destroy the earth and Pink diamond did not agree and since it was her planet
and don't forget, a majority of her experience with humans was probably from pink diamond's zoo. their lives are mostly play and superficial feelings, and it's understandable for her to assume all humans are like that.