In public, Hux barely reacts. He is a professional and there's no way he would let his emotions affect his judgement or interfere with the mission of the First Order and their shared goals with the Knights of Ren.
In private with Kylo he's a ball of concentrated vicious rage - punches Kylo in the face, twice (once to crack his nose, once to make it hurt), hurls every ounce of anger and vitriol and disgust at him, brings up every painful insult he can that he knows will cut deep into Kylo's own insecurities - makes Kylo suffer.
But finally, on his own in his quarters, he just curls up and quietly cries, because damnit he really loved Kylo...
(Bonus points for Kylo being genuinely sorry and regretful about what he did - maybe he'd been justifying it with some force related nonsense that he now realises is nonsense - but knowing that's not going to cut it to make things right again)
(Up to the filler if they want to give this a happy ending, just make it hurt first)
Kylux breakup: Hux's threefold reaction to being cheated on
In private with Kylo he's a ball of concentrated vicious rage - punches Kylo in the face, twice (once to crack his nose, once to make it hurt), hurls every ounce of anger and vitriol and disgust at him, brings up every painful insult he can that he knows will cut deep into Kylo's own insecurities - makes Kylo suffer.
But finally, on his own in his quarters, he just curls up and quietly cries, because damnit he really loved Kylo...
(Bonus points for Kylo being genuinely sorry and regretful about what he did - maybe he'd been justifying it with some force related nonsense that he now realises is nonsense - but knowing that's not going to cut it to make things right again)
(Up to the filler if they want to give this a happy ending, just make it hurt first)