I feel like not enough of our analysis of Rien really touches on his "indulgence".
It is very open to interpretation, but to me it feels very much like... all the horrible things he did almost felt as guilty pleasures, with only a distant panging resentment. His TRUE emotion at the forefront is gratitude and indulgence. That resentment- literally on the other side of the mask- is of course made literal by his wound. It is absent in his everyday life, but reopens when in proximity to Ryoshu. When Rien is torturing poor saps in that basement, or even the implications of what he did to Shiomi, that wound remains closed- or in the latter case, hadn't happened yet.
That wound did not exist before Ryoshu's exodus from the House. His symbol of regret, his regret itself- it only became a factor after he was wounded. I do not believe he resented the prescripts before then. It is the distinct mark of how he exclusively dreads that the prescripts drove a wedge between him and his daughter, and yet even then that dread isn't enough for him to change his ways. Because of his indulgences. The indulgent nature of prescripts is what keeps him there. His sadism.
Combine this with his apprentice's whole gimmick being "the will of the prescript aligns with my own", I feel like typically people lend him too much benefit of the doubt when it comes to the extent of his villainy. There's a tug of war discussion on whether or not he is the victim or the abuser- and first off, let's be clear, the fact that people see that as mutually exclusive is telling- but the fact that people gloss over the flavor text that pretty explicitly show there are noteworthy amounts of sadism and masochism in his motives pre-Ryoshu feels shortsighted. Both the pain he inflicted and the sorrow he endured are "indulgent". He is his own victim and he loves it.
This also is very interesting when you consider that it inflicts Karmic Consequence on him, implying this is deviation from his God's intent. Hermes does not want him to find a twisted indulgence in his tragedy, or a cruel love of torturing people. Hermes wants him to be utterly hollow. Ironically, much like the Muga form.
I don't have much of a conclusion from here. It's rather wandering. But I think its crucial to consider as part of his character, especially how it DEFINES his relationship to Ryoshu, Sora, and Shiomi.
Admittedly, this is also the most unsure i've felt on one of my analysis posts. I think im onto something here, but I also just woke up. Let me know if you think so too.
