Details from the book, chifeng wanted to use their brotherhood as a way to guide guangyao. in the very beginning of their relationship, chifeng respected and liked him a lot even promoted him to be his right hand man so he wanted to use their vow to try and kind of steer him/keep him on the right path but he never trusted or rly liked him again after stuff happened in their past
Yea I love nie mingjue. Hes like pure badassery lol and guangyao was always walking that grey line like he'd do shady shit but save a life at the same time or have some silver words to back up said shady actions so mingjue always had a hard time reconciling with him bc he was such a steadfast, rigid and honorable man. He was black and white and he couldnt deal with guangyao's grey scale. Add to that that at the end of the day guangyao was essentially selfish just compounded the problem lol it's hard to explain without spoilers lol
I don't quite get why even when the headless guy, don't remember his name in this moment, was already suspecting of jin guangyao WHY he still did the kinda vow to be brothers, I don't get it!!