Major spoiler ahead.
You've been warned so seriously, don't look if you're gonna be prissy about being spoiled.
Yayul betrayed MC by killing her aunt in a public place. I think he wasn't supposed to kill her at all coz it was what he promised MC. He could have done it in secret but he chose to do it publicly because he knew by doing so, the aunt would mention something about MC being connected to the Heavenly Demon(they're related - I think he's her grandfather). Yayul wanted MC isolated and ostracized by everyone since now people knew her connection to the big baddie. From what I remember, aunt WAS a bad character. But Yayul killed her out of entirely selfish reasons. It wasn't to protect MC. It was so he could have MC all to himself. This after all the hard work MC put into feels like nothing now thanks to that. It was ultimately what broke their relationship apart.
The one Yayul mentioned in the recent chapter when he was asked by the Cheonsan Flame Lord is his mom, Byeok Ki Hyeon.
Spoiler:
She was mentioned as a slave but great in martial arts (famous like Euigang, that's why Chonsan Flame Lord was shocked) from Byeok Clan, although adopted (due to her skills). She was known to be missing for years (bec she was "taken" and got pregnant with Yayul)
I want to know if Yayul will stay with Yeon till the end, or he suddenly went off seeking revenge?