Absolute truths are common among less evolved peoples, especially those from inferior worlds. Such beliefs are rarer in the middle worlds and almost non-existent among people with vast knowledge as they tend to doubt everything.
It can be said that in Blue Star, few things are considered absolute truth by everyone and the Mermaid Royalty's honor is one of them.
Through several generations of Royal Mermaids from the first to the present, all Mermaids of that lineage have put their people's happiness first than theirs and done whatever it takes to protect them.
With incorruptible ideals, the Royal Mermaids have never done anything vile to achieve their goals.
Kaisa's case made many people wonder about that, but in the end, she sent her own soul into the cold darkness to protect her people, just like the second Fox Princess did. Even though they used dark magic for that, they were the only ones harmed by that.