>>109310889>In the past, it was explicitly stated that SUPERMAN gave Batman the kryptonite and it was SUPERMAN that told Batman to kill him with it if he ever went rogue.
Specifically, there was a kryptonite ring that Lex Luthor had. The radiation cost him his hand, and eventually his entire body (which is why he pretended to be an australian bastard son, when he was actually brain-transplanted into a clone body).
That ring got stolen by the scientist who guessed that Superman was Clark Kent. Luthor had fired her because that idea was unbelievable to him. She hired PI's to prove that she was right, and eventually went mad from other events that happened in her subplot.
After deciding to move away from Metropolis, she got mugged and killed in Gotham. Batman found her irradiated corpse, and then an irradiated corpse of the homeless man who mugged her.
Batman found the ring, had a story with Superman about the irradiated corpses, and then told Superman about the ring. Batman gave it away, and the story ended with Superman giving it back, and telling him to use it if things even went bad.