Theron left the Sect by its gates in the dead of night. This action he knew he wouldn't be able to hide, but he didn't want to in the first place.
By leaving now, the odds he would be accused of murder were less. Even if some suspected the Fourth Elder's death, they wouldn't realize the timing of it.
While it was true that Theron was responsible for the Eighth Elder's death, it was a completely different matter if others knew he had such assassination skills. They would only be more wary of him.
The fact he had entered a stalemate with the Fourth Elder made them confident in still being able to handle him for the time being. Of course, they had no idea how many of his own abilities he had restrained himself from using.
For example, with [Entangling Blood Vein Pupils], he could control Gold Mana instead of being restrained to Silver Mana. The leap his strength would take with the former couldn't be understated.
