i think it's both to save shioon from the murims and there is something else and a lot of The Breaker fans speculate that you must have a broken ki center to train black heaven techs (i think BHT is just one tech of Black heaven and there will be a lot more)
Like how the chief tried to use Shion as hostage, other people might also do the same due to grudge of their fallen master/brother or fellow student. Goomoonryong was about to leave to some place so ofcourse he can not protect shion, not to mention he also can not take him along. So it seems it was for the best that he did what he did. I doubt he thought anything about helping Shion master the Black Heaven and Earth technique at the time. After all... He is shot, very tired + beat up and not only that but his woman just got killed. You could compare Goomoonryong to Lighting Tiger from veritas. Both are master with incredible talents and students who start off weak and naive. Both of them thought their student could never improve at an alarming rate. But they eventually did after encountering many life endangering situations. Anyhow enough of that. That's pretty much what I speculate.
My Top 5 Manwa
1.) Witch Hunter (Badass and greedy. Tasha Godspell is an asshole, a good type of asshole)
2.) Veritas (Badass evil and cool. Gangryong will fuck up anyone, even kittens.)
3.) The Breaker series (I just love martial arts.)
4.) Immortal Regis + Chavelier of the Abyss (Good story and art style. Damn! I want his sword, Depore.)
5.) Legend of Maian (So-So in story and art.)