Most of the men and women at Abu Ghraib and the many, many more in such positions did not enter the situations that determined their subsequent degraded behaviors with evil intent, nor in most cases with a unique predisposition towards sadism. The authority given to them within institutional systems whose conditions and environments allowed for the development of a moral and ethical code unto itself is innate to all institutionalized systems that have the availability or even the assumed availability of the use of coercive force. Power corrupts, but power, even over one human being, when force is unilaterally supported by the assumed seat of power, corrupts rapidly, decisively and potentially contagiously.
You have likely seen this study, but worth the watch. There have been many more such studies on human behavior in authority, but this is well worth watching again and pondering.