Why does an act of consent make such a moral difference, that an act that would be wrong taking a life without consent, is morally permissible with consent?

为什么征得了同意，就能在道德上如此不同呢，没有征得同意就杀人，在道德上是不允许的，征得同意就是允许的吗？

耶鲁公开课 - 公正课程节选