OK, it's not a masterpiece, and I don't either think it claims or expects to be one, it is clearly low-budgeted and flawed, but it does its job: entertain. Actually, the setup part is a little too long, it could have been done better and more effectively without many useless, slow paced scenes, getting straight to the point and using more time to analyze the characters and the relationship between them.
But when it does start, wow... this is torture porn at its goriest. I'm not a huge fan of blood and gore, but I liked the concept behind this movie. The message that I got from it is that, whatever bad a human being might be able to do, taking revenge, an eye for an eye, simply means lowering yourself to his own level of inhumanity. We do have law and punishment for a reason, not to masochistically give up any rightful feeling of revenge we might carry inside ourselves.
The slaughterer turns into the victim, the victim turns into the slaughterer, what is the difference between them at the end of the movie? Just one, made by Derek's final decision, which I won't spoil here. But it's no big deal, Derek is not human anymore, anyway. Not anymore than his victim.
All in all, philosophy apart, it's definitely worth a watch, if you can stand hard torture scenes and can overlook the bad acting and low budget setup.