I will never understand how humans can treat other humans with such cruelty.

Oh man. This does not even begin to define cruelty. Put in context, these were a team ("americans"), that were hit, very cowardly on 9/11 and they believed they had genuine conspirators in their prison. Sympathy they felt for these people wasn't high. That is where their cruelty came from. This does not begin to define cruelty, though. It is cruel but not long ago people would unjustly invade the houses of innocent people, imprison them and do even worse things to them, for no good reason.

Just for being slavic in Germany, you would be sent to a camp worse than Guantanamo bay only 70 years ago. Just for being German in a Soviet-occupied street, you and your daughter risked being raped.

Guantanamo bay was cruel but not "impossible to understand how humans can treat other humans like that". Put in context, most things are understandable, except for genuine cruelty, which happens and is done for the sake of being cruel out of disdain.

The US government outlawed torture 10 years ago. Guantanamo isn't unfathomable. Torturing of innocent civilians in their homes, cruelty happens and people should be familiar with it, in order to know how to rid it.