crescentmoon, with respect, those verses you quote really don't show Islam in a particularly good light regarding slavery. Not a single one of them says "Slavery's wrong, don't do it". They merely show that freeing a slave is effectively equivalent to paying a fine - it's not about what's good for the slave, it's about the slave-owner having to pay a price for his/her transgression. It confirms that slaves are seen as property, not people.
As far as I can work out, the bit about slaves seeking a writing is about a slave buying him/herself off of the slave-owner. That's not exactly altruism is it?
as for christianity, the campaign against slavery in the UK was actually won by Christians based on their belief that it was wrong in the bible.
Yet the vast majority of slave-owners in the UK were, um, Christians who based their belief that it was ok on the Bible. If you have a book as vague and contradictory as the Bible to pick and choose from you can find a passage that will support pretty much any moral position.
In America, similarly, the vast majority of slave owners were Christian. The anti-slavery movement was spear-headed by Thomas Jefferson (who was a Deist who did not regard Jesus as divine) and one of the most influential anti-slavery texts, "On the Rights of Man", was written by the Deist Thomas Paine.
But you're right in saying that atheism by itself offers no restraint against slavery. Unlike the Talmud, the Bible and the Qu'ran, though, atheism offers no defence or justification for it either.
But then atheism isn't a religion. It's not there to tell you how to live your life. We're all grown-ups. We can work it out for ourselves. If you need a book to tell you that treating human beings like property is wrong then you need your head examined.