Those who accuse the Bible most often commit a logical fallacy commonly called a categorization fallacy. Most often, they're referring to "man-stealing", which is not only prohibited in scripture, but is punishable by death in scripture, when what they really mean is indenture, a type of servitude in which a person or family agrees to work for a master and be under his authority until a debt is paid or a given time or condition has passed.
This is wrong. So very wrong. Int he OT, the rules for the indentured servitude type of slavery only applied to Jews "owning" other Jews.
But the rules for owning non-jews are as follows...
“‘Your male and female slaves are to come from the nations around you; from them you may buy slaves. You may also buy some of the temporary residents living among you and members of their clans born in your country,
and they will become your property. You can bequeath them to your children as inherited property and can make them slaves for life, but you must not rule over your fellow Israelites ruthlessly.”
-Leviticus 25:44-46
Exodus 21:20 And if a man smite his servant, or his maid, with a rod, and he die under his hand; he shall be surely punished. 21:21 But, if he lives a day or two, he shall not be punished: for he is his property.”
So you can beat your slave as severely as you want as long as he lives at least for a few days. But if he dies after a few days, then you are off the hook because, as the Bible says “he is your property.”
Yes, the New Testament does give instructions for slave owners but, again, it's talking about indenture, not man-stealing.
I think you're mixing up the OT rules for Jewish indentured servants and what the Romans did in the NT. A Jew could only own a Jewish slave for something like 7 years (can't remember the exact time period) unless you give him a wife, then he's yours forever. Roman slavery, the type talked about in the NT, could last a lifetime. Sure there were ways to get relative freedom (although you'd then still be attached to your master's household as a lower class citizen) but the master never HAS to let you go after a certain time period. Slavery in the new testament wasn't about paying debts, it was slavery.
EDIT: I found the verses about Hebrew slaves:
" If you buy a Hebrew slave, he is to serve for only six years. Set him free in the seventh year, and he will owe you nothing for his freedom. If he was single when he became your slave and then married afterward, only he will go free in the seventh year. But if he was married before he became a slave, then his wife will be freed with him. If his master gave him a wife while he was a slave, and they had sons or daughters, then the man will be free in the seventh year,
but his wife and children will still belong to his master. But the slave may plainly declare, 'I love my master, my wife, and my children. I would rather not go free.' If he does this, his master must present him before God. Then his master must take him to the door and publicly pierce his ear with an awl. After that, the slave will belong to his master forever. (Exodus 21:2-6 NLT)"
"
When a man sells his daughter as a slave, she will not be freed at the end of six years as the men are. [WHY NOT?!] If she does not please the man who bought her, he may allow her to be bought back again. But he is not allowed to sell her to foreigners, since he is the one who broke the contract with her. And if the slave girl's owner arranges for her to marry his son, he may no longer treat her as a slave girl, but he must treat her as his daughter. If he himself marries her and then takes another wife, he may not reduce her food or clothing or fail to sleep with her as his wife. If he fails in any of these three ways, she may leave as a free woman without making any payment. (Exodus 21:7-11 NLT)"