There's a bit more context to it with the politics around it.
Basically, Rose started her rebellion on the idea that the Gems should be allowed to be and do what they want, rather than sticking to their strict role driven society (like, you're made to be a construction worker, you can not be anything but a construction worker).
This extends to ALL gems in her eyes.
Meanwhile, Bismuth, a construction worker who chose to be a blacksmith, hates the "upper crusts" with a passion because they shatter (kill) the rebels
Bismuth makes a shattering weapon and shows it to Rose in private
Rose says no shattering, Bismuth gets upset, ???, they end up fighting, Rose poofs and locks away Bismuth.
It's a big mess of people making the wrong choice really.
But I still agree that they shouldn't have used the shattering option.