What responsibility do the bystanders of the world have for the atrocities committed?

That would probably depend on how much they knew about the atrocities before they happened, and whether, knowing about them, they would have been able to shorten their duration.

Whatever their responsibility it wouldn't reduce the responsibility of the perpetrators of the atrocities.

I might have a guilt to bear for what X did to Y, but I didn't cause or encourage them to do it. They are 100% responsible.