The origin story might be rooted in a particular event where Lex Luthor thought he could control Kryptonite better than he actually could. He might have been using it in an experiment or a plot against Superman. But things went awry, and the Kryptonite started to affect him negatively. Superman, who always stands for justice and the protection of all life, saw an opportunity to show his moral superiority. He saved Lex Luthor from the Kryptonite, hoping that this act might change Lex Luthor's ways, or at least prevent a worse outcome for the city and its inhabitants. This act was also a testament to Superman's ability to rise above personal vendettas and do what is right for the greater good.
In the origin story, it could be that there was a third party involved. Maybe a new villain emerged who was more dangerous than Lex Luthor. This villain managed to get Lex Luthor in a situation with Kryptonite where his life was at stake. Superman, realizing that Lex Luthor's death at that moment might lead to chaos or a power vacuum that the new villain could exploit, decided to save him. Superman knew that Lex Luthor, despite his flaws, was a known quantity in Metropolis. By saving Lex Luthor from the Kryptonite, Superman was also making a strategic move to keep the city's power dynamics in check. It was a complex decision that was influenced by Superman's long - term view of protecting the city and its people.
One possible origin could be that Lex Luthor, in his never - ending quest to defeat Superman, miscalculated while handling Kryptonite. He put himself in a position where the Kryptonite was going to harm him more than he expected. Superman, being the hero he is, sensed the danger not just to Lex Luthor but also to the surrounding area. Since Kryptonite can have unpredictable effects, Superman intervened to save Lex Luthor. It was a moment where Superman's heroism trumped his animosity towards Lex Luthor.