The big mistake they made with this movie

Was the depiction of the gang leader. A couple of things he did I found rather dubious:
a) that he brought his girlfriend to be gang-raped
b) that he couldn't fight another man one-on-one

In my experience, a gang leader never behaves like this. It would have been more realistic if one of his subordinates had brought the girl, and if he (the leader) had prevented his gang from attacking the older man when he intervened.

One thing I learned about antisocial people higher up the hierarchy: they have self-control. They are aware that they are the toughest wolf of the pack, so there is no need to cause violence unless absolutely necessary. They also have the ability to influence others who are not as adept at controlling themselves. That's why they are the leader.