How could anyone find something so horrific as funny?

I watched Roll Red Roll on Netflix and I can’t shake it. It is a nightmare. It was about the Steubenville High School rape case. High School kids partying. A 14 year old girl just got wasted and passed out. Several of the boys raped her and performed other sexual acts on her unconscious body. They carried her to two other parties where the abuse continued.
Some of them appeared to find it just hilarious.
WTF is wrong with them? How could any human being find this to be casually funny? How could any human being even DO such a thing?