Well, 'pokemon killer fanfic' may center around a concept where there's a being or a situation that's endangering the Pokemon. It could be a story written by Pokemon enthusiasts who want to explore a darker side of the Pokemon world. It might involve a powerful and evil trainer who has the goal of eliminating all Pokemon for some twisted reason. The fanfic could then follow the journey of other trainers or characters as they try to thwart this evil plan and save the Pokemon.
I'm not entirely sure as it could vary a great deal. It might be a fan - made fictional work where the story involves some sort of 'killer' concept within the Pokemon universe. Maybe it's about a character who has to face off against a particularly dangerous or malicious entity in the Pokemon world.