This topic is a issue with alot of non believers. So it might be a good idea to be very familiar with what the Bible is actually saying. That way when some one comes along and brings this up again. we can at least have better answers & scriptures , then we had this time.
The problem is there is no good, or convincing answer for an unbeliever.
For a good Biblical answer was given in the following:
There is even more to God's purpose in undesirable slavery.
It's found in what Jesus said Jn 8:34: "Everyone who sins is a slave to sin."
We also learn in Ro 3:9-10 that all mankind is under sin.
So everyone on earth is a slave. . .to sin.
And now we're down to its purpose. Slavery was a pattern, symbol, type of mankind's slavery to sin.
To understand, and appreciate, freedom from slavery to sin, by the death of Jesus Christ to those who believe,
God's people needed the experience of slavery in Egypt, and the experience of being delivered out of that slavery by the blood of the lamb on their doorposts.
After the exodus from Egypt, slavery was still ordained for those outside God's people, that they may also understand the meaning of their slavery to sin, and redemption from that slavery among God's people by faith in God.
After the exodus, Hebrews were not allowed to be kept as permanent slaves.
They could be kept as "hired workers" for a period no longer than six years.
Only foreigners to the people of God could be kept as slaves and personal property.
This showed the difference between the people of God redeemed from slavery by the blood of the lamb on their doorposts, and those that were not the people of God.
The parallel to the people of God in Christ today can be easily seen by those who are that people.