Throughout human history the strong have exploited the weak. It stems from a natural pattern: the strong prey on the weak. However when humans started to actually think about their actions they realized that they could do more than attack the weak and steal their food and shelter. They could exploit them. They could force them to serve them and do their work for them. That's the extra bonus we got from being intelligent. Unfortunately, when this exploitation mentality is left unchecked it leads to our current predicament.