
I think that Isshiki-senpai was really motivated by the effort and time that Kinokuni put into trying to master what he has done in a genius short amount of time.
It's only so long before a genius gets tired of the praise and what he can do and then gets bored and demotivated to do anything worthwhile.
He decided that he would probably advance the "traditional" cooking of his family line by making unconventional twists to it and probably got disowned for doing such things. But by doing such things he managed to use his talent to put wild game into a delicate dish.
Now that's talent used right :D
I think that one split personality came to life when Yoseob questioned if he did kill his mom or something else. As for his dad, he probably died of alcohol poisoning or something.
Yoseob is mentally unstable since young and not having a good environment and people around to be able to support him through his growth, and he's always very silent about things, conditioning since young, and he should learn to express himself more so he can get the help he needs.
Also here I'm not going to say that he's the only one at fault, but maybe Jaeha is too, in some way.
But with the latest updates, we can tell that that other Yoseob is a part of himself that he cannot control at will.
It's like that thing that's all inside of us, waiting to emerge at the right time and consume us, but we are able to control it properly, (by not acting on it)..
His dad died because he brought home that pesticide bottle he called medicine. He probably mistook it for alcohol and drank it
Ah, I see. Thanks for the enlightment on this matter.