I think you have to understand Myeong knows the brother for as much time he knows Geon. He was a baedong, and the brother is older than him so you can see he looks up to him like an older brother, following his advice.
Besides, it's the the first time in a long time Myeong is with someone from outside the palace, he even said he hasn't had a proper conversation in ages. He is still a kidnapped man clinging to something from outside
Even if the brother was against Geon's torture, he couldn't do anything because they were direct orders from the king.
The brother is still a piece of shit for making Geon go to the younger brother and ask for forgiveness, but Myeong doesn't know any of that

I kinda hate everybody now. After knowing of changgeon (I forgot how to spell his name!) past, I feel like everybody around are hypocrites. Even Myeong. He kinda sucks. He jumps at the brother’s arms as if the dude is such a noble and rightful human being? The dude acting as if he’s so high and mighty that the kingdom will fall naturally in his noble hands? Fuck them all. I wish Geon just burns the whole kingdom to ashes. I don’t care about the romance or anything anymore. He deserves so much better than all the assholes around him.