There is always conflict over how society ought to be run
If men were angels, no government would be necessary. If angels were to govern men, neither external nor internal controls on government would be necessary. In framing a government which is to be administered by men over men, the great difficulty lies in this: you must first enable the government to control the governed; and in the next place oblige it to control itself. Alexander Hamilton
At no time in the history of the world has any government running a country of any significant size done everything right. In fact, it is not possible to do everything right, because to do that, first you have to get everyone to agree what "doing everything right" even means. But people value different things. The various interest groups that make up said nation will find that they do not agree on what is important. So it is impossible to get everyone to agree.
Say the trapper's guild and the artist's guild are in a disagreement. The artists say that the trappers are killing too many animals and ruining the forest, so they can't draw/paint/sculpt their works of art anymore. The trappers say hunting might not bring in as much money as the artists, but they bring in a lot of food and make much better warriors when called upon to defend the kingdom than artists.
No matter what you do, there is no way to make everybody happy here. Any society is full of conflicting interests like this, and balancing them all to keep the peace is a large part of the job of any ruler.
Consequently, there is never any shortage of people who feel like they're getting a raw deal from the current regime. There will always be those who believe they have a better idea, and if only they were in charge they know they could prove it.
It will not be difficult at all for these people to link up with the hunter heirs and build a coalition to make the kingdom a better place. As for a moral justification, that's easy. If the witches are the legitimate government now, then they got that way by overthrowing the previous regime and thus are not in any position to complain that someone else is better at it than they are. And, of course, if they're illegitimate, then that's all the justification you need.
Look at modern politics: Fundamentally the different parties have differing views on The Way Things Should Be, and they fight it out. They just don't literally fight it out anymore. Society has evolved to the point that people who lose these fights don't lose their lives, but it was a long and bloody struggle to get to a point where that actually worked well. Policy disagreements have always been around; nowadays it is considered entirely inappropriate to resolve them with actual force, but this was not always so.