Wasn't it stated that Kuma was a king before he became a revo??
As for his epithet, I believe he might've been given the "
tyrant" name similar to how Robin got her "demon child" epithet... as in, marines killing his kingdom and the WG twisting their truth, publicly accusing Kuma for these actions; the marines destroying his kingdom could be related to the CD's... maybe Kuma forbid a CD from entering his kingdom, or maybe he refused to submit to an order by the CD's that was too horrible.
That might've been the trigger that connected him to the revos, and why he joined them; he witnessed a horrible injustice, committed by the celestial dragons... but knowing Kuma was incredibly kind, it's likely that he witnessed his own people being murdered in response to him defying a horrible order from the CD's.
This is all just speculation though...
But as for him having a pirate crew, I'm pretty certain he doesn't.