How can the ones who walk away represent an evil just as bad as those who stay in Omelas?in "The Ones 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:

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” 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:

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"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:   

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"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.