I'm pretty sure, Kwon believed that Shioon would be corrupted by Goomoonryong if he stepped into his shadow again, rather than letting that happen, he gave his life up so Shioon would stop believing in him
@N0VAZER0 is correct, Kwon wanted to release Shioon from his dependence on his master and his love for him. He knew that Nine Arts was too far gone to be a good teacher for Shioon, and associating with him would only make his position in murim very difficult. He died so Shioon could reject Nine Arts.