Have you read about the slave trade that took slave from Africa to bring them to the United States. People were stuffed three deep into ships with no room to even sit upright . About one half of the people brought across even survived. You can read more from this
eyewitness account.
Saying that life was better as a slave in the US then free in Africa is sheer idiocy. People had been living just fine there for thousands of years. There are people who live similar agricultural lives in small villages now, would selling them into slavery be doing them a favor?
If you ever find yourself in a difficult situaion... would you like to be sold into slavery?
The fact is that many slaves died in captivity from abuse, or just from neglect as they lost their value. They were basically treated as livestock.
Slavery was a horrible thing. Many people were mercilessly slaughtered.
To suggest that somehow the slave traders were doing the slaves a favor is ludicrous.