According to the author, the fate of the Elder Priest was supposed to be explaining Eastern views of rigid social hierarchy and the pitfalls of being greedy, penance for the priest murdering MC's parents, and wanting to get into some unique sexy scenes. The first two reasons make sense, but it's poorly done; the EP is a side character so the social commentary isn't very present, even with the main characters, and the EP was threatened with his life to kill people and the emperor and new EP were part of that, but they don't get comeuppance (yet).
Honestly given the author's other work, I think they just have a fetish for torturing old guys. If they said that, it'd still suck, but it'd be upfront. The reasons they gave don't completely feel finalized.
Man I'm just fucking disappointed. Like ending was rushed and author just has a whole side story of rape. Wtf.