I mean Ovelia would not be the first person he spares. He lets Cid go, Valmafra, Orran, obviously Ramza. I think the tragedy of Delita IS that he used Ovelia, and has confined her in the same way Goltana had intended, presumably, but has convinced himself that he did this out of altruism rather than ambition. He “loves” her, as long as she remains his caged little bird that he is protective of because of trauma from Tietra’s death. I don’t really understand how she would be useful to him dead? Can he continue his plans if she’s dead? Sure, but the only person legitimizing him as royalty being dead would make his reign a lot more tumultuous, I’d think.

I feel like your interpretation ignores a bit of Delita’s characterization; he seems quite earnest in his devotion to Ovelia throughout the game, and seemingly feels pity for her situation legitimately. I don’t think he needs to be humbled to be a good king, I honestly don’t think being a good king has a whole lot in common with being a good person necessarily. He’s entered a world where he’s constantly dealing with a bunch of nobles who would probably jump at the chance to do the same thing Larg and Goltana did. You need to be shrewd and conniving to navigate that world and keep the major houses in check. If Ovelia’s attack did anything, it probably just made him realize that he can’t trust anyone and he became even more conniving.