My archaeology professor made an interesting point while we were looking at Athens today. The definition of a tyrant has changed over the years. Today, it means some merciless dictator nutjob type who can't quite start his day without killing civilians. During the first millennium BC, it did not necessarily mean cruel or deranged. A tyrant was someone who seized power peacefully but without any sort of order to it. No voting, no nominations, no discussion among royal houses. The person just says "mine" and takes the power and goes about his rulerly business like normal.
With that in mind, I ask you, is Bartholomew Kuma truly a tyrant? He was a Revolutionary, and those guys don't seem like the slaughter aimlessly types. Evil and revolution against a corrupt world don't seem synonymous to me. If you're wondering what I'm getting at, allow me to answer that question with a question. How did Kuma become Shichibukai? There is no way the World Government would be willing to work so closely with a hostile man who stood for everything they were against. However, Kuma is not necessarily weak, even before his modifications. He is twenty feet of silent mean with a built-in pressure cannon and the ability to manipulate that pressure however he sees fit, whether that was sending someone on a trip a trip around the world, or blowing crap up with a pose resembling the Kamehameha Wave (if you don't know what that is you should be ashamed of yourself).
Now, the part you've all been waiting for, my hypothesis. We all know that the Shichibukai is composed of pirates,with the exception of Kuma of course, and the government tries to keep it as pirate as possible. Sengoku even said that the Shichibukai needs infamous pirates. So, why pick Kuma, a not pirate? What would you think if I said they maybe didn't have a choice? Kuma could have simply declared himself a Shichibukai when a spot was available. He probably sent a polite, formal letter to Sengoku that said something along the lines of:
"I call dibs on the open Shichibukai spot. Does anyone have any problems with that? If you do have problems, it's in your best interest not say anything."
(Shichibukai) Bartholowmew Kuma
OR, the more heroic idea, he met with Sengoku, or Tsuru, or the Gorousei, and told them very plainly, "I am going to be a Shichibukai, I'll play nice and follow orders as long as you play nice with me. Good day, sir" (Sengoku: But, but... Kuma: I SAID GOOD DAY(movie humor)).
And that's how he became a Shichibukai, by taking it tyrannically. I'm going to guess the government sent out some lies about how they chose him because he was a tyrannical pirate, the bounty could have already existed, or they could have given his 296 as a starter bounty, as anyone with the balls to do what Kuma might have done deserves that much. As for what he did, that was probably more government spread rumors. If they can say Nico Robin sank 6 battleships, then they can make up stuff about Kuma. That's my idea. Prove me wrong or tell me otherwise.