The Resistance wins and storms the First Order's (Snoke's?) headquarters. They get to what is basically the dungeons/cellblock part and start freeing the captives.

One or some of them (can be Poe, Finn, Rey, Leia, redeemed Ben, Luke, just about anyone) happens upon a cell far, far in the back of the building. And inside, they find a very broken former General Hux (who mysteriously vanished after Starkiller blew up back in the day).

It's clear that they did a real number on him, violent abuse (possibly also of the sexual kind) and all and they can't help but feel a (misplaced?) kind of pity for him.
And so, instead of taking him into custody to have him tried for his crimes, they just quietly let him go.

(Would prefer if he survived though.)