cause the other demons will try to hurt gyeongjum if he stays with him and cause he’ll ve the reason gyeonjun dies if they stayed together
But I thought gyeongjun was bound to die anyway since he’s a door and he absorbed all that angelic energy
actually gyeongjun's background is very questionable. his eyes for example are peculiar and we've seen some type of power release a few chapters back. the only reason why cain is stepping away is because he's not strong enough to protect gyeongjun while at the same time he doesn't know that gyeongjun may be more capable (possibly not completely human)
I’m not really grasping the situation right now but oh lord Cain’s face when he had to leave Gyeongjun why do you have to separate why can’t they work something out