I think what makes it worse is that the Prince knew that coming to this world; she would know nothing about the mannerisms and culture and social customs of the country. Yet he continued to shelter her and not let her gain experience by being with other women. Even when Sei isekai-ed Elizabeth was the one to teach her not to but her feet out or show too much because of the customs. Especially since in Japan, Japanese people are accustomed to being less sensitive about their bodies (communal bathhouses for everyone). It is very important for other women to be around each other because we mostly learn by copying, but Aira only hung around guys. Which in this case, the men in those kind of worlds are very open while women are not. Especially when it comes to jokes and stuff, so she must have thought that it was normal to be that close with other people, however, she only hung around guys because the Prince made her follow her around like a dog. WHICH is another problem. When Elizabeth and Aira first meet in the hallway, where Elizabeth mentions to Aira that she could help her study, the Prince comes out of nowhere and interrupts them. What makes matters, the way the Prince unconsciously talks to Aira is like that of training a dog. When he said, “Aira, follow.” Like he was speaking to a dog. Istg I have never been so mad. He thought of her as a lost puppy, and with his social status, I’m 100% sure he thought of her like that.
If he didn't ignore Sei he would have put her in similar situation to Aira. Stuck in a lower level, covered in promises and assureness that her bad reputation is ignorable because once she save the country everyone will feel horrible and be grateful to her. Extra protected Sei wouldn't have seen the dreadful situation of the kingdom and would have been in a situation where she was told to take her time and there's no worry while in the meantime soldiers are dyng in the front lines and there are several monsters in every region. Being out on a pedestal is a detrimental. As such he actually did a terrible job.
Also read the spinoff and watched the anime. Knowing that he was made aware 2 girls that were summoned the next day of the summoning but decided to ignore it despite multiple reports. Proves he feels entitled that his position and view is above facts. And this denial continues for several months until he was finally put in a situation where he could no longer deny it. He decision to take the blame and be indeed just bratty Prince is mainly to save Aira's rep. He continues to NOT acknowledge that Sei is the saint even after. As he stated he continues to believe he didn't make any mistake as of today and continue to support the idea and his position that Aira is the Saint. He does not believe he is in the wrong. And he believe based on the his feelings and sympathy for Aira. He decision to play the role and be bratty Prince is also based on his personal preference and support of Aira. Altho done with good intention to help Aira later.
His support and action is based on favoritism. And bruh, soldiers and fighting and being heavily injured in weekly/daily confrontation with monsters. He knows what's happening outside the castle but decided to prioritize his favoritism of Aira thus equally view 'his people sacrifice' ignorable and prioritize his bratty Prince facade and Aira's future (one person) over many people. Like bruh you aren't really thinking of the 'people~". It's awful but despite he did it for Aira he would make a terrible king as he prioritize his circle of friends and liked people than his kingdom. Saying he acted and was playing the role of bratty Prince right at the start is UTTERLY contradicting HE CHOSE AIRA OVER SEI for weeks even before Sei achievement which become known months later. Thus he had no reason to feel the need to put a facade since Aira's rep as saint was not endangered by Sei yet.
The guy is actually equally a terrible Prince too. From Aira's spinoff for months they would receive notice of Sei's achievement and the reports that they are 2 girls summoned but decided to utterly ignore it. In the anime only after Sei subjugation he finally start reading reports about her and he could no longer sustain that Aira is the saint. Both him and HIS AIDS are bad. The aids utterly fail to convince him of otherwise until it was too late. The Prince for ignoring opinions that are not his own.