I believe that the root problem is this: there are many within the police who truly believe that the ends justify the means. In other words, they might readily accept that things such as the Reid technique, bullying in interrogations or overt threats are "extreme" acts, but that if they result in getting the "bad guys" convicted, then all's well and good.
The problem, of course, is that these sorts of interrogation techniques, while they may indeed have some effectiveness against bona fide criminals, also have an alarmingly high tendency to get innocent people to say incriminating things. And this in itself renders such techniques not only unethical (and unlawful in most jurisdictions), but also essentially worthless as a means of gathering evidence. After all, how can one judge whether the person "buckling" (copyright: Perugia Police Chief de Felice) is a real criminal telling you the truth, or an innocent person reacting adversely to the extreme techniques?
But the problem remains (in my opinion) that many within police forces around the world - and especially in, shall we say, less-enlightened jurisdictions - honestly believe that extreme interrogation techniques are a valuable and entirely valid tool. On top of that, of course, there is a very small minority (in my opinion) of police who believe they have the right to commit more extreme misconduct - either in order to "get" a suspect whom they truly believe to be the culprit, or sometimes (and even worse) in order to manipulate charging/conviction rates. And there's an even smaller minority within the police who are intrinsically corrupt and mendacious, many of whom also positively enjoy inflicting psychological and/or physical abuse upon people in custody.
Fortunately, in the UK the Police and Criminal Evidence Act (PACE) has effectively codified acceptable police behaviour, and has delineated exactly what can and cannot be considered reliable interview-gathered evidence. As a result, police malpractice in this area is now extremely low, and on those rare occasions where there is potential malpractice, the courts will use PACE to negate it. If only such codification or legislation existed in the Italian criminal justice system.......