I'm 80% sure it is about the sister. Maybe she died and he got do mad, but idk why the MC wouldn't figure that out if that were the case.
I think it's related to what you're saying. I have a feeling that Baek-gyeon was trying to use the "reversal" technique that the mc is searching. As you know Baek-gyeon was already there when the mc was going to the cave and I think Baek found the technique in the previous life.
I may be totally wrong but this is just my gut feeling.
Ohh! That’s a nice theory!
I just want to know why Baek-gyeon betray Hyun-ho. Seem like there a story behind it.