I feel hxh is pretty realistic by not letting it's characters have a black or white morality.
Oito is also not on a morally higher tier than others. She basically sold herself for fame and fortune - and now regrets it. You should also not underestimate the feelings that come along with motherhood - seeing another child getting killed does affect her of course and Momoze is even Woble's halfsister. Mito might have reacted the same (even tho she's not a mother by biology).
Furthermore helping Momoze would have ended for her either being locked up as a culprit for her death, being locked up for violating the rules to not enter the chamber or being killed too - there is nothing she could have done (neither her guards).
Speaking of her wanting to save Momoze is also not morally better for me than Gon jumping after a random sailor on the way to the exam. He risked his own life for a stranger and he probably didn't have to. Also in the same moment he was saved by Leorio and Kurapika who interupted their fight to help - I think none of them calculated to get benefits from that concerning the exam.
Melody's goal is to save the world from the sonata that killed her friend and cursed herself - she's not intending to get it to be transformed back to her true form if i remember right.
Chrollo as a child lived for values of equal access and participation - he did nothing for himself and still doesn't by sacrificing himself to keep the children of Meteor City safe. Of course the path he took is far from being morally right but at least he doesn't act for selfish reasons and tries to create a better world.
Sheila resisted joining the dark side because it's most likely contradicting her moral compass and she gave her dearest treasure away to thank others for saving her life.
There is definitely more to it but for what we know so far she has high moral standards.
Just a few examples why i personally wouldn't value Oito higher in that regard than others.
(Can you tell me in what chapter Melody is sure Kurapika would kill her? That wouldn't make any sense for me since he is capable of much but he's not Sasuke
I think Oito is. She did mistakes in the past, but what she did with Momoze is exactly why she is moral.
Her biggest motivation is saving Woble, so motherhood and protecting her child.
But she endangered her own child to save someone who is her enemy (Momoze).
I don't any other HxH character acted against their own interest like that, leave alone to that extent.
Some of the other characters have good goals, but they are all connected to their personal story.
Melody was hurt, so she wants to hunt for the sonate.
But that is a personal motivation, not morality.
The only hunter who (we know) hunts for a moral goal is Leorio. His motivation is connected to his backstory, but he legit actually wants to study to save people as a doctor and he understands that this is not just connected to him wanting to be a good doctor, but also to money. Leorio understands there is a bad system in this world and he wants to act to make it a little a better. That is a moral stance.
Melody does not want anything like that. Melody does not use her ability for the good of the world or anything like that. Melody is not standing in an Emergency Room to tell doctors about whether anyone's heart sounds funny. Melody hunts for something that, let's face it, is not going to kill a lot of people if you need to hunt for it to find it.
Aside from Leorio, I do not think there is a single hunter who ever intended to use his abilities for the good of humanity as his biggest motivation. Some hunters do good, but they tend to do it because it interests them.
Kurapika killing the spider would probably be a good thing for humanity, but that is not why he does it. He does it because they hurt him.
And I think what Oito did is far more of a moral stance than Gon jumping after a sailor. Gon is a daredevil and was pretty sure kurapika and leorio would catch him. He endangers himself for fun all the time and is insanely strong. He did it because he felt like it.
Oito is a woman scared out of her mind, surrounded by monsters with the life of her baby on the line and she still saves an enemy under the risk of her and her babies life.
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