Read the novel some time ago, well at least part of it... This is akin to a 70s low buget german porno, the "plot" only exist to push out the sexual content. I don't mind sex but it has to serve the story and not the other way around.
He basically is saying that he would fuck that girl if she were older, treating her like a child. Ok. And then he encourages her to kill people who restricted her freedom. Like wtf? You either acknowledge her as a child who is not old enough to murder, have sex and be independent, or you acknowledge her as adult and she is old enough to do all of that. You don't selectively treat her as a child here, and treat here as adult there. Not to mention that if she really attempted to kill that bandit by herself it would end up very badly for her, but somehow she magically does it and returns unscratched.
You seem to comment based on the presupposition that killing -whether it be in self defense or for vengeance- is something for adults only. Given the context of the story, this idea that it can only be 'both' or 'neither' for sex and murder seems unusual. Basically, if you want the ML to fuck the murderloli, just be honest and say it, instead of trying to make some weird logic statement about it, lol.
if you are old enough to kill adults you are old enough to be considered adult. Because that requires competence and a mind with a strong adult-like will. Also, the MC is the one who is honest with himself when he says "What a waste" after she leaves. It's like someone indoctrinated him into feminism and he acts against what he truly wishes, which makes no sense for era when gladiators still existed. Back then teens were considered small adults, not children.