Boredom, peer group pressure, power imbalances. I have watched (and been) a good person caught up in the group hysteria that crushes some people. It's shocking.
I was recently listening to a podcast about the Stanford prison experiment - 1971 - that documents the descent of good people into doing bad things:
"In 1971, 23 American college students' lives were changed by the now notorious Stanford Prison Experiment. For the eminent psychologist responsible, Philip Zimbardo, the parallels to the atrocities at Abu Ghraib are palpable. In an exclusive Australian interview, he joins Natasha Mitchell, to reflect on the capacity in all of us to commit evil. It's a case of good apples put in bad barrels."