Alex Gibney is interviewed by Robert Scheer of TruthDig as I reported here Gibney describes the phenomenon of force drift whereby torturers no longer see the subject as human and keep applying more and more pain.
The military are well aware that this happens which is why there are rules to control this behavior. The Bush administration removed these rules, however, and the consequences led to the torture and death of an innocent young Afghani.
Many of the interrogators in the movie Taxi to the Dark Side were young National Guardsmen and Military Police with little training. Several of them are haunted by what they saw and what they did. It is not hard to see how the same thinking that leads to force drift in the military can also take hold amongst civilian police forces. Unchecked force can be blamed on local police chiefs and will ultimately lead to force drift.