Most of players I DM wouldn't think twice about cutting the horns off a minotaur they captured as a way of protecting themselves against it having a weapon.
If one of the players was captured instead of killed by a group of orcs for instance, I would have no qualms about doing the same.
There are so many spells and traps and monsters and situations in 5e that do horrible things; I'm not sure why cutting the horns off a minotaur is labelled "dark" or "horrific."
The Heat Metal spell burns someone alive inside their armor that they cannot immediately take off.
An illithid uses its face tentacles to extract and eat someone's brain.
As a player, I'd much rather have my horns removed than be cooked inside my chainmail or have my brain be someone's dinner.
The minotaur could try to have metal horns made that are better than the original.
I think you are over-reacting to something that should probably be happening if a minotaur was taken alive by evil creatures with any kind of intelligence.
IMO, it all depends on if minotaurs are "monsters" or "people" in your game.
If they are monsters, I would expect players to treat them similarly to animals . . . if they are dangerous it is OK to kill them, if they need to be captured for some reason, it is okay to declaw, dehorn, or otherwise "make them less dangerous" . . . as long as it isn't done cruelly.
If they are people . . . big difference. Removing the horns from a person (minotaur, tiefling, or some other horny race) is a violation, a disfigurement, and is an evil act. Even if, over time, those horns can grow back.
Of course, in the real world, humans commit evil acts all the time, often justifying them in the name of safety. How your players deal with captured foes can make for some good roleplaying with interesting moral choices. If I'm the DM, and the PCs capture and dehorn a minotaur . . . that minotaur (or maybe his friends) is going to show up later with a pretty serious grudge against the party! More story!
Personally, I find playing D&D today a bit troublesome when caught between a mythic perspective and a more modern "sci-fi" perspective. When you consider "monsters" from a mythic perspective, they are usually violent and evil and must be destroyed to protect the village. But in modern sci-fi, we have all sorts of monstrous-appearing races that are just different kinds of sentient beings, people, and should be treated as such. And of course, we can have both perspectives at the same time. In your campaign, minotaurs, tieflings, and dragonborn can be people, sentient beings deserving of respect, no more-or-less inherently evil than any other race of people. But at the same time, mind flayers might be corruptions from the Far Realm that are intelligent and sentient, but not truly "people", rather a corruption of beings that used to be "people".
We are all going to handle that divide a bit differently in our games, but I think it's worthwhile to do so mindfully and not use the game as an excuse to act out in ways we wouldn't in the real world.