We have vain brains; we see ourselves as better than we really are. We like to think that we exercise free will, that put into a situation where we were challenged to do something we thought unacceptable then we'd refuse. But, if you believe that, then you are probably deluded.
I make this claim, based partly on the work of Milgram devised and carried out ingenious experiments that exposed the frailty and self-delusion that are central to our lives. He showed how easy it is to make ordinary people do terrible things, that "evil" often happens for the most mundane of reasons.