Yeah, from a slave's perspective, it is not merciful. But they were slaves. It was merciful for a slave. Not merciful for a servant, but merciful for a slave. Non-Christian countries treated their slaves far worse. God said it was okay to beat a slave, but not to death, AND NOT CAUSE PERMANENT HARM. Permanent harm is rather easy to do, so beating slaves had to be done with restraint. We also have to judge the practices of that time by the standards of that time. Back then, beating a rebellious child was not frowned upon, whereas today, many people call for spanking to be made illegal.
Captives from war were absolutely enslaved. If they were men, and if they survived the war. There is a good chance that men of that country were fighting in the war and thus would have been killed as enemy soldiers, and were not taken captive. Those who were, were treated as slaves as I have described. However, women and children were treated very differently. A captured woman that a man wanted to take as his wife, would be given a month to grief for what she had lost, and then the man could take her as his wife. That seems barbaric by our modern standards, but remember that back then, women had almost no rights and without a man to protect and provide for her, she would be doomed. Women were given the chance for a new life with a home and a provider. If the man decided not to take the woman as his wife, she was not to be sold or to be treated as a slave (Deuteronomy 21:10-14). Children were given similar treatment.
I don't understand why you are bringing up your other points. Personally, I believe that the New Testament replaces the Torah (although a lot of the Torah is reiterated in the New Testament) but you don't have to agree with that for my points to stand. I never even said that Sunday was the new Sabbath. I said that many Christians who do not observe the Sabbath rest on Sundays, which is not the same as the Sabbath. As for Roman Pagans rewriting the word of the Chosen, why bring that up here? I don't see relevance to this debate. We are talking about what the Bible says about slavery. The Anti-Christ has nothing to do with that either. Seriously, I have no interest in debating these other topics. I'm saying that the Bible does not justify slavery (specifically the Atlantic Slave Trade of the last few centuries) and it does not encourage excessive beating of slaves. Please stay on topic.
I encourage you to view the link I referenced before coming back.