The Enemy will insist that Bolin joined Kuvira and the Earth Empire late in the game and wasn't that intwined with it. But I, an intellectual, firmy belive that Bolin heard about Kuvira and Baatar Jr heading to bring order to Ba Sing Se. And he rememebers the view from the airship of Ba Sing Se- the city his father grew up in, the city he had tied so many hopes of family and companionship to- burning, in chaos. And he remembers having to carry his grandmother out of her home that was going to burn down as she protested because it was her home. And he remembers seeing himself in his younger cousins as they huddle in the corner of the airship after losing their home. And he wants to, so very, very badly, to be useful and to do something good and meaningful, especially for the people who are his only blood family in the world other than his brother.
And it's really a no-brainer to make that choice despite everyone in his life telling him to do otherwise.
