The word slave comes from the Romans. When they conquered most of Slavonia, the word Slav since then has become synonymous with people who are subjugated with slavery. Slavs were given to Roman warriors as a reward for their on field performance. They were also called addicts, which is a Latin word. Someone who is a slave has not control over what he does, the same is said about addicts when they are controlled by drugs. Your often controlled against your own will, that is why your a slave of your substance. Slaves of other kind can also be called addicts. This might also have influenced the word Slav by which many eastern European nations are called by.
Many other civilizations used slavery without much thought about it. But the Romans were known to be very cruel, that is why they were conquering lands after lands, they would sell the slaves to warriors or roman civilians. The slaves where treated badly. Many knew this before getting caught so lots of defeated Roman foes choose suicide over a life long of slavery.