A small theory on ma head's been popping around that maybe Tiel might end up solely brainwashing Taegyu instead of Thomas, and when the ice dungeon arc ends, he would try to join Suho's guild so that he can no longer be traced back to any incidents regarding the Reapers.
I mean, thinking about it, 1) he manages to get closer to Suho, —sure they may have doubts at first for suddendly trying to join a guild, especially one where you can't really do much since Suho is the one raiding all the time anyways. But he also doesn't need to convince them much since the Reapers conveniently have no problem with him leaving AND he can add Minsung's attack for a reason why he feels 'unsafe' to work with them. —
2) Since he has only been in the guild for a while, if they are ever found out that they have been brainwashed, they have little evidence to pin point his as the cause since he's been there for quite a small amount of time, and the situation happened before and after he left. Maybe he was just lucky to leave before he could get dragged in as well.
And 3) Having more access to Suho's rutine would let Tiel plan for a day where he is at his most weak in order to fight against him and potentially win. After all, Suho had already made it clear that he eants to fight him at full strenght, so when he does merge with 4 and 5th fragments, the time will come soor for when he does finally reveal himself in front of Suho, and what better way to do that then by betraying him when he is at his most vulnerable?
