When one goes back in history, one discovers that European victors in conflicts collected slaves as part of their tribute. The Romans were large users of slaves taken from their conquered peoples and displayed when the triumphant military leader paraded through Rome. One recalls Pope Gregory's (AD 540-604) comment on seeing the pale-skinned English (Angle) boy slaves "Non Angli, sed angeli."
It is thought that much of Cicero's works were in fact written by his slave secretary whom he later freed. (One hopes that there was not some form of pederasty afflicting the Church at that time.) I am currently reading a novel about the Danes (Vikings) ravages of Saxon kingdoms a few hundred years later. Their slaves were called "thralls" (from which we get the verb enthrall).
Then we can fast-forward to the 20th Century and remember the slave labourers in Germany and the USSR. So we should not be too hard on the Arabs for being slavers as they learned from Europeans.
JE comments: And let us not forget etymology: Slave comes from the word Slav.