Wait who's the old man? It's the reason Jae Shin's Dad got so pissed but what did he do to...

Yes. He's the old man who appeared in the first chapters of the story. He asked Jaeshin to extend his lifetime after gambling with him. Jaeshin granted him his wish but he didn't tell the old man that he would take his own son's life to fulfill his request. When the old man understood wht happened he begged Jaeshin to cancel his wish and take his life instead of his son but it was too late. After that Jaeshin was punished by his father for his actions and the old man is now seeking revenge over Jaeshin. Understandable!