One of Oda's great unappreciated talents is his ability to keep his world and story consistent even as the scope outgrew his original plans by several orders of magnitude. I think there's enough in Thriller Bark to know that Oda always planned for Kuma to be a secret Revolutionary, but I doubt anything but the broadest strokes of his backstory was hammered out until much later. The Eleven Supernovas, and thus Bonney, weren't even conceived until Saboady, and even if you wanted to say that Oda already knew Kuma let himself be turned into a robot because he had a kid, he had to twist the threads of the story to turn Bonney into that kid.
To be perfectly honest, Kuma during his introduction is a whole lotta smoke. All the reader knows is his moniker and that he had the reputation for committing atrocities. That's as close to a blank slate as you can get while still building hype, and that blank slate lets Oda slot in whatever he feels is appropriate once he gets to the point where Kuma's backstory is important. That flexibility is a wonderful tool, but Oda's careful to keep the old consistent with the new, and that's enormously difficult when writing serialized fiction over a period of 25+ years
