I have a feeling the mass spree of murdering the management of the country is going to come back and bite him in the ass. And he seemed like he was smart at certain points previously, why can he suddenly not tell that saying "off with their head" is scaring ppl and not making them admire him? If this was france he would be next in line to be executed. Not to mention ppl will doubt the accusations considering how much suddenly appeared out of nowhere by someone who shouldnt know (though I guess the guards will trust him, as they somehow caught everyone redhanded during the act).
Marrying porter to the princess could either be a really good move, or the worst one. Apart from not understanding how the hell he managed to make them agree to it (its literally a slave), it is also going to make him lose any goodwill he still has from pretty much anyone. But if he can ride out the storm, odds are ppl will see his calming down and distancing from politics as legitimacy to his claims, and porter might gain popularity if the princess at least looks happy.
Though I have no clue how he intends to run the country, now that they lost so many administrators, and the king has been taught absolutely nothing about ruling.
And if killing the royal family brings so much troubles, why couldn't he just say they were exiled/put in a convent/house arrest, and then assassinate them when public eye was no longer looking?