After reading the story I immediately thought that the point was that most of us live in Omelas. I don't think I have it in me to walk away.

I wondered the same thing.

But then I wondered "Why did they walk away? Why not comfort the child and end it?"

In the story she says a single kind word to the child would end it. I can understand living there, but I don't understand walking away.

By walking away you are not helping the child. So what does it matter that you are not benefiting?

To help the child and end it means to deprive many others of happiness as well. It is also likely to be wasted, as she writes in the story, the kid may be so far gone beyond consolation.