When Henrietta adopts Louise as her sister at the end of season 3, she mentions that Louise will be in line for the throne. At first glance, this seems like it'd be mostly honorary...barring unexpected illness or assassination, such a young queen would normally be expected to live long enough to have children, who would be higher in the line of succession than the queen's sister. However, given Henrietta's broken engagement and her grief at Wales's death, she may either intend to stay single, despite Wales's last wish, or at least believe she is unmarriageable due to their tryst being public knowledge. In that case, she may very well be adopting Louise with the intention to actually pass the throne to Louise or Louise's children.
Technically Louise comes very close to being a Yandere in the novels. In one scene, seeing Saito with another girl upsets her so much she hits him with a explosion spell which apparently comes very close to killing him. Apparently, he couldn't even move afterwards.
Actually, just Louise and Saito's relationship in general if you think too hard about it. I mean, in the anime at least, it's implied that Saito being whipped is a daily event, and if you take into account that he still does her laundry and sets out her clothes, and gets whipped if he does complain about it, Saito looks kinda like a slave.
The anime exaggerates her temper more than she is in the light novels. The anime was also directed by the same person as the Love Hina anime, who did the same thing with Naru Narusegawa and made her come across as a total bitch. Why do that to the main love interests? Apparently, the angry girlfriend/wife thing is considered the height of slapstick comedy in East Asia, meaning that he exaggerated their tempers because it was supposed to be funny. Culture Clash, or bad taste?
This troper thinks Louise's antics were supposed to be funny, but it got real bad when Saito had to suffer serious injuries for little to no real reason at all, especially in the anime when Louise got more and more trigger-happy with her Explosion spell and more and more people get hurt for something that is none of their business. I get it that everything in anime is exaggerated, but even if it's true that the angry girlfriend trope is still pretty popular in East Asia, I think that if she's angry at Saito for not giving her enough attention, she should just take it out on him alone with less lethal methods (no whipping and indiscriminate firing) and no one else has to get hurt. Lover's quarrels, after all.
Don't blame it all on Louise. Saito isn't the innocent victim many would cast him as. The man is VERY vindictive in his own manner and him pushing Louise's buttons aren't ALWAYS accidental. In his own way, he gives as much as he gets. One of the reasons for their problems is Saito's immaturity; he doesn't get how screwed up Louise really is because of the life she's lead or the pressures of her society. And he has been known to hang with Kirche and Siesta specifically to get back at Louise by going with girls that treat him nicely instead of trying to work things out.
A bit amount of Fridge Horror for Louise is her being a Void Mage in the first place. For all that the Void is held as a holy element, the light novel has some subtle hints that using it leads to psychological deterioration of the mage that hosts the power. Tabitha's Uncle, King Joseph, for all the psychopathy that he shows in his schemes and actions, is actually an Empty Shell that goes to the lengths he does because he wants to feel something, anything again, and the narration in the Light Novels makes reference to his emotional state being "Zero", which is something used often to describe Louise AND the Void.. It's also implied that Brimir wanting to kill the Elves was due to deteriorating Sanity brought on by intense paranoia to a shadow enemy that may or may not have been real at all. Although unintentional, Louise escalating things with Saito after getting her powers unlocked aligns a bit too well with her slowly losing her mind because her power is actually poisoning her for anyone's taste.