Humans were often called a being potential. Unlike angels who were inherently good, or demons who were evil, humans had the potential for both. While there have been numerous benevolent humans through the years, the reality was simply that people were fucking bastards.
The drop rate of Benevols was a clear indicator of this. I forgot what the actual number was, but I think Cynthia mentioned that it took hundreds if not thousands of regular zombies for each Devil's remains.
People, both Human and Reapers have been killing shit since time immemorial. Whether it was for their sustenance by hunting or just because they wanted to. There was no better way to illustrate how malevolent people were than when they took life.
Maybe this was why we became reapers. We took life. That was both our purpose and our way of life. While it may have been possible for humans to live without killing anyone, for Reapers it was not possible. Not if you wanted to survive in our fucked-up reality.
