In the long run, the oppressor is also a victim. In the short run (and so far, human history has consisted only of short runs), the victims, themselves desperate and tainted with the culture that oppresses them, turn on other victims.
If I was told to sacrifice something to prove my devotion to God , if I was told to do what all monotheists are told to do and admire the man who said 'Yes I'll gut my kid to show my love of God.