Just purifying, actually, but that's enough! Because corruption is what's stopping Hyunmook's regeneration from working as it should. Awakeners usually heal from almost anything except for corruption, more the higher their level (low level recovers slowly from light wounds, middle recovers from a lot, high are reeeaally hard to kill). So purifying him let's him heal himself :) Purification is a REALLY OP ability in this world, especially inside the Abyss

In the novel he doesn't just lose an arm, he's hurt way worse, part of his body gone. And he doesn't say he'll try to get hurt as little as possible but the very opposite: as MUCH as possible, because he wants to make himself as harmless as he can for Yohan so he can't hurt him if he completely loses himself. He was trying to give Yohan a way to escape because he feared what he could do to him while in his most violent insane estate (from fighting the pseudosubway) (/TДT)/