Why did Onesimus return to be a slave? Why did Onesimus feel an obligation to return to be a slave? This seems rather counter-intuitive to me.


From the perspective of Faith, I can't see the difference between human master and human slave.
From the perspective of Secular, Onesimus managed to escape the existing system.
From the perspective of Morality, Onesimus broke an unjust law of humankind.


Thus, what was Onesimus's reason for returning to be a slave?
You have to start by remembering that in the society of 50AD, slavery was considered an absolutely normal and proper part of society. To suggest in those days that slavery should be abolished would be the equivalent of proposing today that money and wealth should be abolished, or that eating meat was immoral. So your claim that "Onesimus broke a unjust law of humankind" would simply not have been understood by those of the time - certainly by non-Christians.

Secondly it has always been a principle that Christians should obey, except in exceptional circumstances, the law of the land. If the government says they should pay taxes, they should pay taxes. If the government says they should keep off the roads at night, they should keep off the roads at night. So in essence Onesimus has an obligation under the law to return to Philemon, and Paul sends him back to fulfil that obligation. 'Escaping' doesn't free him from that obligation, any more than getting away with a robbery entitles someone to keep the proceeds. Moreover, because Philemon is also a Christian, Onesimus is in effect stealing from a fellow-Christian if he doesn't return. 

However there is much more to the book of Philemon than that. Paul makes a great deal of pointing out to Philemon the new relationship between him and Onesimus: as you put it, there being in Christ "no difference between human master and human slave". He strongly hints without ever expressly commanding it, that Philemon should free Onesimus because of this. He hints rather than commands because he realizes that Philemon is fully entitled to keep and punish Onesimus, and he wants Philemon to not do this of his own free will, not because he has been told to.

We will never of course be sure that Philemon followed through with this, but I personally like to think so.