Placing a high value on human life is a quite recent thing
It's not something that suddenly appeared in the 18th century though. Rather it evolved over the centuries. Christianity and Islam did introduce some respect for human life through the concept of universal salvation. In the ancient times lives of most members of out groups were viewed as totally dispensable. During the middle ages that changed to some degree, generally mass murder and or enslavement of entire populations was frowned upon (even if still practiced on many occasions). And heathens were at least were offered the chance to convert before they became fair game.
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It's not something that suddenly appeared in the 18th century though. Rather it evolved over the centuries. Christianity and Islam did introduce some respect for human life through the concept of universal salvation. In the ancient times lives of most members of out groups were viewed as totally dispensable. During the middle ages that changed to some degree, generally mass murder and or enslavement of entire populations was frowned upon (even if still practiced on many occasions). And heathens were at least were offered the chance to convert before they became fair game.