Earlier quoted context omitted.
Regicide, and even just murder in general, is highly frowned upon these days. Back then it was de rigueur when the concept of fealty required reciprocity in the keeping of one's word.
I don't see the connection between your comment and mine?
Today one can lie with much more impunity, at best tying things up in court for years until the person you lied to gives up, at worst being ordered by the court to make good on the promise and pay some legal costs (maybe).
As your synopsis shows, it's not prestige that forced integrity, it's the fact that others heard, and presumably would enforce such vows, even against a king, that forced integrity, even from a king.