I see your point of view, but it's the opposite of Rob Zombies from the beginning.
In horror movies, writers want you to be resistant at the beginning, but by the end lust for blood, even rooting for the killers. In Hostel, people are resisting the terror, led through the suspense of the beginning and then when the killing first starts, they are grossed out. By the end, (SPOILER ALERT DON'T READ MORE IF YOU HAVEN'T SEEN/WANT TO SEE 'HOSTEL'), you are hoping the last remaining guy will savagely murder his captor. In the theater I was in, the crowd clapped at that last scene. I think this is along the same lines, Rob Zombie wants you to lust for their glorified and gresome demise.
Horror movies, in this respect, are the opposite if typical Western movie justice. I don't disagree that it could have been a cool alternate ending if they would have been brutally murdered by the law, in inhumaine ways.. maybe even making that same point of "The Departed" of 'what's the difference?' But, like dorokusai said, I think he wanted you to know that they were very, very, very crazy. So crazy, that, I don't think he expected people to relate with him. It's just more fun to cheer for them.