Well, the chapter name is a bit of a spoiler, lol. This whole concept of 'suppressing evil to become good' is a bit of schlock. If anything, and what has been shown in this chapter explicitly, when the true persona reemerges, the feelings of betrayal and resentment only become all the more intense the longer that the individual was forced to behave. It is a band aid solution, at best. I did see this guy turning out to be a 'traitor' though; this author is very formulaic when it comes to this. All the while during the training montage, Tateshima was the only one who wasn't actively helping anyone improve, instead only dropping a taste of his skill and quickly downplaying his overall abilities. We might have seen a few more hints at his powers when he was nearly pressed to engage evil Chun Li, but Rokudou unintentionally helped him maintain his cover by intervening.
As a side note, it seems incredibly short-sighted that neither Tateshima nor the exorcist had noticed Rokudo's incredible ability to calm Juri Han down with seemingly little effort. The master exorcist should be well aware of the power of the mark on his head, even if he doesn't know its true purpose. Will Erimo succumb to Rokudo's charms, as well?