A secret and evil organization within the Pokemon world could be the antagonist. For example, an organization that wants to use Pokemon for immoral and destructive purposes. Gladion might be in their way of achieving their goals, so they get rid of him. They might have powerful resources and trained assassins to carry out the deed.
It could be a power - hungry Pokemon trainer. This trainer might see Gladion as a threat to their goal of dominating the Pokemon league or the entire Pokemon world. So, they plan and execute his killing.
Maybe a vengeful Pokemon. If Gladion has accidentally wronged a particularly strong and vengeful Pokemon in the past, like harming its family or taking something precious from it. This Pokemon could seek revenge and be the one to kill Gladion in the fanfiction.
A practical answer is Chapter 18, because it turns the idea into a concrete scene. The chapter opens with In the corner, a girl in a black dress sat quietly, holding a glass of..., adds pressure through I have no reason to joke with the rest of my life." "Moreover, I wouldn't..., and ends on Hurry.
Chapter 739, "Not Paying Debts, Shameless!", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event. The chapter gives readers a real answer by opening on Shu Yan isn't that ill, it's just a common cold, no, no need to go to...
Chapter 653, "Everyone's Champion", is the clearest early answer because it turns Everyone's situation into a concrete plot event. The chapter gives readers a real answer by opening on As dawn broke, "Wilderness Survival 30 Days" released a notice on...
If you want the clearest chapter reference, go to Chapter 5. Early on, Actually, when Manager Fang saw Jiang Li, he was somewhat surprised.; later, the chapter leans on Because she was really too beautiful, and very young, completely different....
The best chapter pointer here is Chapter 5, "Catching Adultery Personally". It opens with Actually, when Manager Fang saw Jiang Li, he was somewhat surprised. and closes on Recently, he did suspect that his wife was cheating on him, but due to the....
Chapter 807, "If You Dare, Go Complain Now", is the clearest early answer because it turns Jiang Bufan's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Jiang Bufan had barely eaten dinner, having spent the whole day being...
Chapter 833, "She’s Also a Quick Transmigrator?!", is the clearest early answer because it turns Space Administration's situation into a concrete plot event. The chapter gives readers a real answer by opening on [It's the Yu Family and that Fengshui Family, the Li Family, who...
Chapter 654, "Did Everyone in Team Q Go to the ICU?!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. It opens with All of a sudden, all public opinions burst out. and closes on Afterward, he excitedly ran to his best friend with his laptop in....
Chapter 813, "She Has a Child?!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter turns that into something concrete by moving from In the past, he might have helped Meng WanZhi speak.
Chapter 670, "Sister Will Support You", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event.