Au where Ben and Hux are soulmates, who meet during a battle between the First Order and the Resistance. The whole soulmate recognition thing happens. Ben panics, because his soulmate in the General of the First Order, and oh Force, why?

Hux is much more pragmatic, and fast thinking. The reasoning is thus: his soulmate is on the opposing side. Hux has no reason to defect. Therefore, Ben Solo is the one who is going to have to change sides. And until Ben Solo is convinced of this, a forceful relocation is called for.

So Hux has Ben kidnapped, locked up in a Force suppressing room, with little contact with the outside world until he can be made to see the truth. Hux is fully aware that this may result in Stockholm Syndrome, but again, he is pragmatic.

Because just because he has a soulmate doesn't make Hux a good person.

Bonus:
+ Between his soulmate being there and his own troubles with the Dark side, it is very hard for Ben to fight as hard as he believes he should.
++ Even in his messed up plan, Hux is very protective of Ben: only the best for his soulmate, and anyone who treats at him poorly will be put on sanitation duty for the next decade.
+++ Leia and Han are involved, somehow.