Correct me if I am wrong. If that rock is a real Kryptonite, shouldn`t General Zod, Ursa and Non be more protected, otherwise they would react in the same form as Kal-El (a.k.a Superman) by the rock`s radiation, you know, get weaker and die. Because the way you portraied in this amazing art, it looks like the enemies are coming towards the Kryptonite to get it for them. I loved the colors and the way the characters are displayed.
The explosion of Krypton resulted in a radioactive discharge that rendered its fragments toxic. That's why it can hurt him...OK, kryptonite can kill everything provided you're exposed to it long enough. It just works faster on Superman because he's from Krypton.
Luthor's had his own problems with kryptonite, wearing a kryptonite ring for so long that he got inoperable cancer from it and had to transplant his eyes and nervous system into a cloned body to stay alive. And yet he learned nothing from the experience, shooting up with kryptonite steroids in the hopes of killing Superman in mano-a-mano combat. I guess learning from your mistakes isn't an option when you're a megalomaniacal sci-fi Hitler wannabe.