Back in the time Brendol Hux was still perfecting the Stormtrooper conditioning program and young Cadet Hux was in a rebellious phase, Hux fell in love with a particularly empathic stormtrooper.
Hux Sr. found out about the relationship, placed Hux's lover under the perfected reconditioning program, and had him rape and torture his son to drive the point of strormtroopers being only disposable tools existing to obey orders, before executing said trooper like a defective product.
Years later, Hux comes across FN-2187's file for troopers to be assigned to Starkiller, and instead of putting him under the watch list/extra conditioning, hides the footnotes on the trooper's empathy.
Then TFA happened.

+ Kylo was the only other lover Hux took (and that was despite his misgivings)---at least his father can't get rid of Ren?
+ Hux Sr., upon finding out about defective stormtrooper Finn, already knew what happened
+ outing Hux: "...at least you didn't spread your legs for this one..."
+ Finn (and the others) doesn't quite know what to do with the fact he and the Resistance victory happened partially because he reminded Hux of his dead lover, or that there is an actual human being underneath the persona of General Starkiller