Nope, nope, nope. I'm kinda with icekatz on this. This is a pretty damn drop-worthy event. This just completely undermines any attempt to make Lucius have the moral high ground or sanitize the entire "conquering" thing. Framing it as a "kindness" to quickly kill someone not involved in the plot just makes it worse. If that's their idea of kind, what's their idea of normal? And this is supposed to be us reading for some budding romance between this man and an abused woman who apparently will go along with whatever he says? Yeah, no.
And to those insisting "it was an act of kindness" no. No it was not. People talking about "but she's royal family what if..." but before the king pulled his assassination attempt, Lucius was perfectly willing to let the former royal family live after their surrender. This whole arc was about conquering Bipka peacefully without bloodshed. A way of sanitizing the whole conquering thing which is kinda important in making a romantic comedy out of such a setting. Murdering the daughter of the man who plotted against him and "killing all the royals just to be sure" erases every last bit of that. And again, framing it as a "kindness" just makes it all the more egregious. You can sympathize with someone doing horrible thing when they're shown to feel like they have no other choice and are conflicted about it, but doing this horrible thing and going "omg, she's so kind!" just indicates a level of brutality/sociopathy that's flat out disgusting, and at heavy odds with the tone of the rest of the story.