Wolf –
“and this was in a time when weak and/or foolish kings very often found themselves dead at the hands of others who would like to rule instead.”
Just to prove your point, Herodetus in his history notes that Xerxes, aka Achashveirosh, was killed by his guards in a coup.
HaLeiVi –
I believe it was a decree, or more accurately, law. Achashveirosh sought council “k’dat ma la’asot im Vashti”. He wanted to know what he could do legally.
Dovid Hamelech –
The text mentions nothing about Achashveirosh killing his advisors.
And before you start talking Midrash, remember, Chaza”l wanted Megillat Esther written in this form. Midrash could have easily been included (and if you are so inclined, you can look at the version of the Book of Esther in the Septuagint, where the author lifted sections of the Midrash Rabbah in order to make the book seem more “religious”).
The text leaves it ambiguous as to whether Achashveirosh killed or banished Vashti. Regardless, the law was that she could not serve as queen and someone else had to be chosen.
Throughout Megillat Esther, the text is poking fun at the concept of “law” in Ancient Persia. For example, drinkng was according to the ‘law’, but what was the law? “Ein o’nes” – no one is forced, i.e., there is no law.