At this point it's an old question, but unfortunately it's one we need to keep asking because some people persist (and insist) on not getting it: what makes someone a “monster?” Unfortunately the default answer tends to be “they're different from me;” they look different, they sound different, they believe in something different…any perceived difference is often enough to label someone as “Other.” And when that gets entrenched deeply enough in someone's beliefs, it can become difficult to make it clear that usually the real sign of a monster has nothing to do with looks – it's the steadfast belief that “different” equals “evil.”
Those beliefs all have their own roots, of course. In the case of Dix Perdix, his determination that monsters that talk and emote are just more fun to kill comes from the “curse” of his forefather Daedalus. Dix tells Bell that all of Daedalus' descendants are born with his directive in their blood: they must complete the Knossos Labyrinth and fulfill his goal, no matter what. The question then becomes whether or not there's really some sort of magic coursing through the blood of the Daedalus line compelling them to follow his will or if it's a belief that's been passed down along with the inherited eye that can open the way to Knossos. If just one person succeeded in implanting the idea of a family curse well enough that his descendants passed it down to their descendants, it could very well exist – but less as an actual curse and more of one the family imposed upon themselves. Dix saying that he feels his however-many-greats-grandfather telling him to work on Knossos could very well be a sign of abuse and resulting mental illness, or just plain old inherited mental illness, and not a curse at all, at least not in the magic sense.
The problem with that sort of this is that they're all in your head, meaning that getting them out isn't an easy, or even a possible, thing. Dix finds refuge from his in acting out – how much do you want to bet he was the kid who pulled the wings off of flies and then watched them crawl around, gleeful that they were under his control? Because that's basically what he's admitting to Bell in this episode: he enjoys hunting the Xenos because it gives him a thrill to know that something with the power to express its emotions is his to harm and kill as he pleases, because what's the problem? They're just monsters. Nowhere is this on display more than when he almost giddily takes Bell back to where he's got Wiene tied up and rips the jewel from her forehead. (It's clearly part of her body, as the strings of attached…something attest.) He loves hurting her, forcing her into a transformation of his volition rather than hers, and he's almost definitely getting off on the fact that hurting Wiene also means that he's hurting Bell, a human who is otherwise legally beyond his touch. And then when he releases Wiene, now in a sort of winged snake form, to the surface, he knows that things are only going to get worse – for both Wiene and Bell.
And boy, do they ever. Wiene is quickly attacked by Loki Familia, who are taking things at face value: a monster has escaped the Dungeon and is attacking Bell and threatening townsfolk. They have no other way to take things, because Ouranos has kept the entire Xenos thing under wraps, only letting a few Familias in on the secret. Whether Bell remembers that in the moment or not, it's still a moment where he has to pick a side or at least be perceived as choosing one. And no matter how much he admires Aiz, she's not the one bleeding or hurting like Wiene is, although we could argue that by keeping her in ignorance she's being manipulated much as Wiene is.
Ultimately this episode comes down to a question of belief: do you believe that monstrosity is determined by appearances or actions? Are you willing to listen when someone tries to change your mind? Who are the bells of Argonaut tolling for? Are they a warning – or a knell?
Is It Wrong to Try to Pick Up Girls in a Dungeon? III is currently streaming on Crunchyroll and HIDIVE.