Ben Solo is struggling. The Dark Side is calling to him, he has clear anger issues, his relationship with his parents is complicated because of them.
He fights for the Resistance because he has been raised by them, and distantly, he believes in their ideals, but they feel very distant at times, which in turn makes him feel guilty because he feels he should be more dedicated.

Ben Solo is a mess.

The thing that helps him cope? Meeting the General of the First Order in secret, acting a true sub and just letting go of all control during their time together. His need for punishment, attention and surrender all delivered by the Resistance's public enemy number 1.
The guilt just keeps on growing. Especially since he has no desire to stop.

Bonus:
+ Hux is a good dom, but not quite benevolent. He gets a kick out of doing all he does to Ben, would be very much in favor of bringing him in the First Order. For power, and also because then they could continue what they do much more freely.
++ Not just pain play: humiliation, feminization, corsets.. There are many kinks to choose from.