Kuma’s Devil Fruit has the Power to defeat Saturn
I theorize that every time Kuma helped someone with pain they suffered at the hands of the World Government, he collected it into an extremely compressed bubble which he carries around with him at all times. At some point during his activities as a Revolutionary, Kuma found out that the person that attacked him right after he ate is devil fruit is one of the Five Elders.
Kuma also had a hunch that he will at some point end up as a cyborg slave once his time as a warlord is over, so he asked Vegapunk to program him in such a way that he delivers the bubble to Saturn should he ever meet him again at Mary Geoise.
Now in some upcoming chapter, Kuma will appear in his beaten down and badly damaged state on Egghead due to some other protocol that made him fly to the island. This will cause him to meet Saturn again and trigger the attack. Seeing the tiny bubble floating towards him, Saturn assumes that this is simply all Kuma’s dying body can muster and will make a point about not dodging it to humble this pathetic attempt at his life. However, the collective pain of thousands of people across multiple decades will simply be too much for his body to handle and take him out.
The main drawback I see with this theory is that it seems like Kuma has been a very virtuous person throughout his life and the title “tyrant” may have been government propaganda to tarnish this reputation. Collecting the pain of thousands of people to deliver it to one person specifically might be out of character for someone like this. Kuma also only met Saturn once to singling him out as opposed to e.g. his former slave master could be weird. However, the flashback is not over so there is still room to speculate.
*Theory by mondian_
