Lonius.. it allows only when lives are in danger..and the economy of a society is in danger.. slaves before were more happier than today freelance workers.. there was a huge population of slaves..Owners were encouraged to free their slaves..You know what..slaves used to cry to leave their owners as they were so close and their owners treated them as family members (cuz of religion) so slavery which religions practised was in concept of adoption. Considering their situation in the arab word if religion skips slavery there would have been a huge deficit in their economy with more homeless and jobless people.