Responses
That wouldn't help. It would just lead to long civil war and dark mages would thrive in it. But I do hope that Duke warns them (Erin, ML and the young knights) before acting like he is coming to Erin... or something like that.
It would be better if Duke stays inside as an allied person, as long that is possible. Duke probably still have to sacrifice himself.
(Dunno how story goes, I haven't read the novel if there was one, can't remember.)
If I was the Duke, I would have killed the king right there, sacrifice himself for a better future for the people.