I think that there can be different ways to view what the people who walk away represent. On one hand, it can be argued that the people who leave Omelas are morally superior to those who stay. The ones who walk away represent a group of morally bound people who cannot exist in a society that seeks to cause pain on a targeted group or individual. In this case, the child's suffering is too much to bear for those who leave. They can be seen as representing those individuals who wish to take a stand against social cruelty and indifference to the suffering of others. Their response is to leave and not be a party to this.
I believed this representation for quite a while until I taught the short story this year. I was taken aback when a group of my students believed that the ones who walk away simply represented a different form of self interested evil in that they leave to placate their own guilt. In this paradigm, the ones who walk away represent another type of selfishness in that they only leave because they cannot fully deal with the guilt caused by knowing their happiness comes at the cost of another. In this light, the ones who walk away represent another form of selfishness that masquerades itself as "honor." There is little they do to help the child other than leave, and in this, they represent a selfishness that might be a bit different than those who stay in Omelas, but it is still a way to avoid the fundamental issue of helping the child who is the subject of abuse. I am not fully sure if I can accept this, but I do see their point. In trying to ascertain how LeGuin seeks to represent those who walk away, I think it's important to draw as broad of a spectrum of representation as possible in order to fully gauge the implications of the narrative. In this light, I think that one could argue that each representation could be a possible understanding of those who walk away from Omelas.
What makes this story so interesting is that Le Guin really searches into the psyche of people who, for all practical purposes, live in a perfect society. Perhaps Le Guin was unwittingly influenced by philosopher and psychologist William James, who wrote:
” some people could not accept even universal prosperity and happiness if it depended on the deliberate subjugation of an idiot child to abuse it could barely understand”
This theory postulates that individual thoughts should guide their actions, and that truth is the consequences of belief. It is this ultimate factor that influences "those who walk away from Omelas"
Le Guin focuses on religion, too, which plays a part in the decision of those few who actually walk away. When you think about the good vs. evil and moral compass, how can a person live with their 'happiness' if it comes on the heels of a child's suffering? And, since the narrator is unreliable (not omniscient), how do we as readers know with absolute certainity that the child feels or understands nothing of its circumstances?
So, the part you're referring to is when an adolescent sees the 'it' for the first time, or an older adults who might think about the child--without acting for years--turn away from the town and leave:
"ahead into the darkness, and they do not come back. The place they go towards is a place even less imaginable to most of us than the city of happiness. I cannot describe it at all. It is possible that it does not exist."
As Le Guin points out most people from Omelas are happy and guilt-free, and, as the unreliable narrator explains, the people may have been "shocked and sickened" by what they first saw in the child, but eventually realized that to release 'it' their happiness would be effected:
"that would be a good thing, indeed; but if it were done, in that day and hour all the prosperity and beauty and delight of Omelas would wither and be destroyed. Those are the terms. To exchange all the goodness and grace of every life in Omelas for that single, small improvement: to throw away the happiness of thousands for the chance of the happiness of one: that would be to let guilt within the walls indeed
In the end, those that achieve true enlightenment are the ones that walk away after realizing their happiness comes at a considerable cost--that of an innocent' child's existence.