What I’d like to point out is that this fellow is actually a nice guy for what Oda showed us. He was presented holding a bible in the first place, and proceeded to save Luffy’s journey 3 times afterward (Thriller Bark, Sabaody prime delivery and Sabaody Sunny guardian). But more importantly he was a member of the Revolutionary Army and, as we know, they tend to not like the World Government.
So the question is: why The Tyrant?
FOOD FOR THOUGHTS:
He was not. Since he was a Revolutionary, in order to be a spy (as a Shichibukai) he needed to be known as a proper pirate. In other words, “The Tyrant” nickname and it’s related stories were just false rumors spread around by the Revolutionaries in order to achieve their goal of having him as a spy. Or, if Oda wants to be darker, it could be that those “evil” actions were actually committed by Kuma, but always in order to be a spy (the ends justify the means stuff ).
*Theory by AName