Wow, just... wow.
We went from comically pathetic mysoginists to... male Mother Gothel turned up to eleven.
Seems pretty clear that she was either locked up because she disobeyed, or because she rebelled (the difference being, the destruction of the capital being either under orders before she stopped following them, or as a consequence of fighting back).
Either way, since he hid her very existence as his daughter it was easy to just take her old actions (done under his command) as proof of her evil and to become a hero to everyone.
Which... makes it extra creepy his actions about the princess earlier.
Doubt it was out of loyalty for the crown now. I'm guessing that with his greed, he plans to groom her and eventually marry her, becoming an eternal king.
And for that, of course, he needs her to be safe from any external harm or ideas so she also stay brainwashed'y loyal to him, just like he had done to Laplace centuries before.
I am guessing the only reason he did not do so before may be because she is the first princess from the royal family in all these centuries that was not too low on the line of succession to not become suspicious if everyone before her was... ineligible. Or the ones that fit, he was not able to get their trust enough to try this.