When Sean chose to join quicksand, it was because Han Fei wanted to build a "better Korea" with "Fa", which was the original intention of quicksand and the reason why Han Fei persuaded Sean at that time. After joining quicksand, he faced many dark things, but he still just wanted to help South Korea find a way to become stronger. After Han Fei's death, this road no longer exists. After the demise of South Korea, quicksand left South Korea and became a killer organization. Although quicksand is still secretly looking for the cause of death of Han Fei, it is not the quicksand that Sean dreamed of at first. He and quicksand have already had great differences. But Sean wouldn't betray his friends, so he left quicksand and found another place.
I think Sean finally joined Confucianism mainly because of Han Fei. Although Han Fei is a legalist, he studied Confucianism and was greatly inspired by Confucianism. After he returned to North Korea, he became friends with Sean, but I think Han Fei is more like a teacher in Sean. He gave Sean a lot of ideological enlightenment, so Sean was deeply influenced by Confucianism. When he needs to find another stable place, Zhang Liang will first think of Confucianism.