What makes it so obviously disturbing? I get that we don't practice it anymore but we are the same culture that used to and we never got conquered or religiously converted by an external culture, did we? And you said that most of our neighbors, including Gylruv, drew heavily from our ancient culture and see themselves as partial successors of it. When did we become "generic westernized empire"?
The "generic westernized empire" is typical for a post collapse large and highly advanced empire. In OTL that was Rome, in our timeline it was Ymaryn AND Rome.
So what happened here was the values mentioned:
-Honorable Death
This was the foundational value of what made suicide appealing.
It was first severely shocked when we double suicided against the Khan, we did a FULL mass levy in the narrative and basically every adult who adhered to that philosophy died in the fighting with great honor.
The deathblow to the value was consolidating around our urban centers, where being willing to die for your job is no longer a social positive. Intellectuals and artisans willing to die over a matter of honor costs society the vast amounts of resources needed to train them. Its a trait that was doomed to irrelevance outside of highly martial cultures where the gain in fanatical deathseeking warriors and especially levies outweighed the costs.
-Greater Good
This was the 'pure' fanatical form of the sacrificial beliefs. This is the value conflated with the actual human sacrifices, because we pulled the value from Xohyr, which is now considered part of Greater Ymaryn(which means there ARE historical records of horrible sacrifices going on there, where said writers probably didn't bother to make it too clear that they were foreign because its self evident fact to them).
We lost this to trait fusion, but repeatedly picked it up, so the cultural record is very confused.
-Personal Stewards of Divinity
Lost/Downgraded in the fracture, since the divine nature of land held by the people is evidently disproven.
Noting that SOME part of "finding new fields" is literal, when we trigger Stewards to get free cultivation actions thats excess people being shed/banished from their home villages to set up new ones.
-Greater Justice
Almost certainly degraded in the fracture. The stabilization of the core lands REQUIRED horrible injustice to be carried out to preserve the Few at the expense of the Many.
Noting that Greater Justice also makes it Just for a scapegoat to take responsibility for a crime they didn't commit and then honorably suicide their way out to protect their organization. You may recognize elements of this behaviour in Korean/Japanese CEOs taking responsibility for some product defect or incident and resigning.
So you know, of course it looks horrifying and alien to the modern Ymaryn. The main values which promoted such behaviors could not survive urbanization and civilization fracture.