The thing is,the ruling class had bad apples,some trully rotten ones,but to then extend the punishment to the close family members,even the extended family,and apparently up to three generations worth of the family,for the actions of one individual?
The revolts were trully barbaric,acted out by working class who saw nothing but blood,even if it was just a little girl who didn't know better at the time.
The rationale is that because the position of power in a monarchy is heredity, to ensure the end of it, the immediate inheritors must be eliminated. Any surviving possible successor could be made into the standard around which the revolution's enemies will gather. Even harmless dumb kid princes or duchesses, as long as they could be propped on a horse by some manipulators, they are dangerous to the revolution, since the enemies may claim some sort of legitimacy and complicate a lot of things on top of the challenges of managing the country the revolution just grabbed.
You can always abolish monarchy,but for the love of peace,do it peacefully through a referendum where the people can democratically choose who they want.
Oh yeah, and the monarch and the noble class who hold all the land, wealth, laws, privileges, and armies will just simply drop it all once a democratic referendum's result was not in their favour, right. So nice of them to just easily and peacefully hand the kratos over to the demos like that.