Same question/concern. I feel like he used it as an excuse to not empathize with others. Maybe because he's royalty that he chose to do so. Because he's been alive for years/decades, why would he still believe that the others don't feel pain like he does. He himself knows he doesn't know what pain feels like, he knows there's something wrong with him. And he chose to ignore how others could possibly feel.
So like OP said, it might be a reason but it's still not an excuse to be a tyrant and merciless as the emperor.
Well, I guess if you can't feel pain, you will never experience how awful it is, so you can't quite empathize with others who feel pain, because you don't even know the concept of pain to begin with. So the king now is learning to empathize through experiencing emotional pain (having seen Heun broke apart & losing him in the end). It's also worth noting that the king described his hurt feelings when Heun chastised him, as his heart freezing to the point of breaking. Kinda make sense, for someone who doesn't know physical pain to describe the feeling using temperature. Though he really should not know how it feels like to freeze or to break, imho.
The point is, he probably know there is a thing called pain. But he doesn't know what it is coz he never experiences it. And he is a king, being brought up sheltered all his life, he probably doesn't have a lot of exposure to see suffering. Try to put yourself in his situation. There are certain things that you just have to learn in life through experiencing it. Take for example, a certain rich & powerful someone whose face is always orange, who has lived a life full of privilege. He never has to go grocery shopping even once in his life. So in his ignorance, he said you need to get you ID checked when you go grocery shopping. So out of touch with reality he was, he could never empathize with the average people who has to worry about paying bills & day-to-day real life worries. Does he not know there is suffering? Of course he knows, he just can't empathize with the average people coz he never has to go through life that way. Something like that, if you can see my point. And sorry for my choice of illustration, I don't want offend people, but I can only think of him to use as an example to use at this time.
They should be able to feel some extent of pleasure, maybe not quite like how normal people perceive it. Like you know, how pain can transform into sexual pleasure for some people. Having this condition doesn't make them numb to senses. They can still feel light touch, like running a feather across the skin for example. They just can't perceive pain. But then again, I'm not a doctor or someone with the condition, so I can't tell for sure... ╮( ̄▽ ̄)╭
Okay, so there it is. The king was born with congenital insensitivity to pain. It's funny how the author picked a real & rare genetic condition as the basis to create the character of the king. Not to be confused with the generalized loss of sense brought about by an illness like diabetes, people with this condition can feel everything (touch, temperature, etc.), except for pain, or extreme stimuli that cause pain. Lots of you have asked why the king is brutal. Well, turns out he never knew what pain was to begin with. To him, breaking an ankle, or stabbing/flogging someone is not a big deal, because for him, it's probably no different from feeling someone giving a tap or hard slap in the back. If you recall back when the king broke Heun's ankle, he said "Had I known you would be in so much pain, I wouldn't have done it". That line infuriated everyone, coz duh, why wouldn't you know something so obvious? Now we know, he really didn't know it. This is one of the root causes that makes him over the top heartless. But having witnessed the process of Heun breaking apart & dying of suicide 100 yrs ago, he started to learn to empathize. And he is still continuing to learn in this lifetime too. But as expected, change can't come overnight, it comes in stages. At least for now, I'll give him a bit of credit for his willingness to learn. And finally, this inability to feel pain is not something to be envious about. Being able to feel pain is essential to life. If you are robbed of that, you are basically being given a very severe handicap & you have to be extremely vigilant about the state of your well being at all times. And no, real people with the condition don't turn into a psycho like the king. They have their own portion of suffering too, although not in terms of physical pain. This is just fantasy story after all. ╮( ̄▽ ̄)╭