reasonable crashout, yes.
killing the crown prince is fair, not so much.
the villain here is the late king. the current king share the blame too for giving the poisoned tea
(because cmon. a royalty who just believe in everything while living in a political hell?).
crown prince didn't do anything.
but again, reasonable crashout.
even if Duke didn't kill the Prince, he is weak enough to handle worse time after that.
do you think people would trust A prince from a stupid and weak king that have been chosen by Evil Late king as rightful ruler?
people going to riot and country would be split apart anyway. having Violet rise to succession is the best way for country and his revenge.
The great successor is Violet, the magic genius,.the very thing of Late king hate and reject sit on his Throne, while his descendents of magic less that he loves, perish. it basically pissing on late king grave.
Prince is rightful successor, he need to be removed to so Violet be the Queen as she is next in line after Duke dies.
Plus it king stupidity that cause Duke loss his child and wife. "yeah, my father hate my brother, even forces him give up his right as heir so i can become next king. he wanted to give secret give to my sister in law. guess that must be a good think". his stupidity and incompetency cause death to his own sister in law and niece/nephew.
a child for a child. the prince is innocent? so do Duke unborn child and wife. its fair for Duke took something from King as he did took something from him.
oh, about lily territory suffer famine but no help come kingdom because the King give help to his close crony first. basically family that close to late king were given first priority while other domain were sidelined.
had the Duke become King, they wouldn't suffer such fate. all of this reveal when Duke plan work out will make people abandoned Royal family and Chose and support Violet as she is student of Duke and acknowledge as Genius Mage. he wanted revenge but not at cost of kingdom falling to ruin as people are innocent.