Maybe he wasn't a traitor before. like I suspected, but if Ye Han now starts to feel alienated, he might listen to the advice from wrong people. In the end if he has spent a lot of time out there in the wilderness while growing up, he would naturally grow apart from both the Ye family and city life. Some dark guild members, who have likewise lived outside of the city, could appear sympathetic and sharing the same burdens. If they could convince him that a great monster uprising is coming in the near future, and nobody can do anything about it, possibly they could get him to do things he otherwise wouldn't.
Although I still primarily stick to my old suspicion of there being something wrong about him. It's just too convinient he was nowhere in sight back in Nie Li's old memories of the invasion, and this time as well only arrived after the dark guild attack had happened. Not to mention he didn't arrive with the other man, but instead had an excuse ready. For all we know, that other person has been dead for a while already.