I've got my own theory on Kuma and Vegapunk'stories.
Kuma used to be a well-known pirate that went to the NW, and that the WG made an outlaw - that's why he had a 296 millions bounty. And this old Kuma, nicknamed the " Tyrant ", is not the one we know.
Indeed, Kuma lost his freedom in the NW when he met Vegapunk who beat him and put an end to his journey : that's how he turned to be one of Vegapunk's guinea pigs. To me, Vegapunk is a scientist that captured pirates to make experiments on them.
Then, Kuma became the scientist's first cyborg, the first one that managed to survive the treatment to become cyborg - and that is because of his unique strong body. That is also why all the Pacifistas look like Kuma : actually they are clones of Kuma as only Kuma's body can endure the cyborg treatment.
Vegapunk managed to keep control on Kuma who still had a free mind because his cyborgs - that is to say the Pacifistas - need a fuel to live that he is the only one to produce ; don't forget that Franky can't live without cola.
Later, Vegapunk made an agreement with the WG : in exchange for his knowledge and technology, the WG provides him with all the things he needs to work. In the treaty, Kuma was decided to be turned into a Shichibukai because he used to be the " Tyrant ", a heartless and scary pirate.
That's why Kuma is the only Shichibukai that is the " dog " of the WG - as Oda said it - because he can't live without his fuel. That's how, Kuma the Tyrant, a selfish pirate who only lived for himself, became a slave of the WG. Despite that, Kuma decided to fight the WG : even he had to obey them, he could still do things secretly as he had the right to travel (he is a Shichibukai after all) and had informations about the scientist division. He got in touch with the revolutionary army and became their spy and their friend.
And you guys know the end, a time came when Vegapunk and the WG decided to turn him into a complete Pacifista and erased his memory.