you can understand it and still disagree with it. rationally I would argue killing him would have been the better choice. The way it is now he could figure out a way or get someone to help him lift that curse and be a nuisance again.
Kinda like with Batman and the Joker. The Joker causes so much death and destruction but Batman just keeps throwing him into Arkham Asylum just for him to break out again and kill some more. Obviously for story reasons it has to be that way, but just through a utilitarian lens at some point someone should've killed the guy, be it Batman or the death sentence through the justice system. (and I say that as someone against the death penalty IRL, but super villains are a different story)
You've probably never met a sociopathic narcissist or else never realized what drives a person like that. You're describing what a normal person would do or feel. People like Bifrons aren't normal. People like him can get obsessed over toying with and abusing specific people who can't get away from them, usually close family. It feeds their enormous ego and gives them an anchor in life that they otherwise wouldn't have, because most people faced with their anti-social behavior simply stop interacting with them.
When they lose this anchor, they can't just replace it with another. To begin with, it only works because they've made it up to be something irreplaceable. Even if the person is dead, they'll continue to obsess and imagine what-if scenarios solely about this person. Literally the rest of their life will fall away. That's why often the most abusive ex-spouses don't find other people to abuse, they keep chasing their ex down, and if they can't they tend to break down entirely.
The Joker, when written by someone who actually understands this, fits the mold perfectly. In stories where Batman has died or appeared to be dead, the Joker becomes at first uncontrollably angry at everything and everyone, then finally listless and unmotivated. He never finds a new foil. Common sense and societal expectations says he probably should have been killed or put away, yes. Society will never allow otherwise in real life. But common sense only applies to common people. The actually rational reality is if Batman and Joker actually existed in real life, and Batman died, I would be fairly confident he'd never be more dangerous than a random drunk or petty thief ever again.
Bifrons won't even get as far as finding a new toy and realizing it's distasteful. He's most likely to end up sulking and moping in some bolthole for a lifetime, if he doesn't kill himself or find someone who will first. There's even a good chance he'll go find some excuse to talk to Nephteros, either coming up with a half-baked solution to the curse that he knows doesn't really work, or else reasoning to himself he can say something hurtful to her that will be worth his death, and kill himself that way. Extreme narcissists are pretty good at rationalizing stuff like that, especially to themselves.
