In Monster Reunion, Centipeetle told Steven about The Great Diamond Authority use their weapon, Corrupting Lights to corrupt her and other Crystal Gems
It's unknown whether or not they still have that weapon but what if when Steven and CGs confront the Diamonds, The Diamonds use Corrupting Lights to corrupted CGs (Sans Steven) and Steven realize he must fight them since his healing power is not enough to make them uncorrupt
How would you feel of seeing Garnet, Amethyst and Pearl become a corrupt gem?