@1idd0kun The WN
@Alex-dono-sama
mentions that since the 1st queen was a lower ranked noble, the nobles forced the king to take a higher-ranked noble consort that suited his royal status. Then the king had to do so to stop potential dissent.
@Alex-dono-sama
That is correct. The king was already yandere for his 1st queen to the point he'd technically confined her in the palace. And since his daughter looked so much like her mother, the king was starting to project his yandere desires on her. Fortunately the king is ill and dying at this point, so he won't be too much of a threat.