I think a lot of people forget the beginning of the story, Yi Zaha used to be the crazy demon and went back in time through that one thing he stole from the demonic cult. Moyong Baek became the poison demon after his whole family was killed, the ones he loved, through the great rakshasa (or however you spell it) killing his loved ones. He experimented poison on himself when his vast amounts of medical knowledge wasn't enough. He ended up saving Yi Zaha in their past life by pitying him, the last bit of humanity that Moyong Baek ever had he used to save him. Because of that one charitable act, Yi Zaha remembered it until this regression and is using it to prevent the same misfortune from happening again. AKA he wants his comrade who also went crazy to stay sane and protect not only himself but his family from any sort of misfortune like that happening again.
Don't bs with terms like that, it's a valid interpretation( when they talk there's always a blooming effect, which is commonly used in romance stories specifically when the love interest and mc interact) and some other things could make you see it that way. I respect your personal view, and ask that you do the same to others who don't think alike.
You obviously have reading comprehension issues. Look at all the comments before mine, everyone is saying that there's bromance between Yi Zaha and Moyong Baek, I was saying that Moyong Baek has a wife so there isn't bromance at all. Obviously your homophobia instantly gave you a rage boner that you couldn't help but beat in the comments. Take your dumbass hateful self out of the comments and keep reading your brain-dead big breasted fanservice manga that you love.

LMAO I love that so many people love queer elements to their stories... Sadly I think people forget that the doctor has a wife and I'm pretty sure kids as well.
Also did the art change a bit recently? I have had a feeling but I feel like it was really a smack in my face these past few chapters. Even the storytelling feels a bit different.
EDIT: I had a feeling people forgot: Moyong Baek became The Poison Demon because of the fact that The Great Rakshasa (or however you spell that old man's name) killed his whole entire family. This is one of the main points of the main story, something very dear to Yi Zaha because while The Poison Demon still had the smallest shred of humanity and pity inside of him he helped Yi Zaha from facing death (i forgot the technical term it was, when his martial arts breaks and he gets those purpley vines on him). The reason why The Poison Demon pitied Yi Zaha is because he saw the same craziness in Yi Zaha's eyes that were in his, so feeling a comradre he helped Zaha and went on his way. This caused Yi Zaha to feel indebted, because even though he's faced those episodes before this was one where he knew he wouldn't have come back and would've definitely died. He ended up remembering it into his regression and wanted to do something for Moyong Baek before he became the Poison Demon, and while he prevented it this time he wants to prevent it once and for all so that Moyong Baek can protect not only himself but his loved ones if need be.
I love bromance, I love BL elements in things ESPECIALLY in martial arts comics. I just know that Moyong Baek has a wife that he loves and cares for. Remember he took on those two women from that one place as """disciples""" or """second help""" because he and his wife had so much work?